Use GenTreeStmt* where it is implied. (#22963)

* Extract `impAppendStmt` and `impExtractLastStmt`.

* Delete `BEG_STMTS` fake stmt.

Use new functions to keep the list updated.

* Retype `impTreeList` and `impTreeLast` as statements.
Rename `impTreeList` and `impTreeLast` to show that they are statements.

* Fix fields that have to be stmt.

* Start using GenTreeStmt.

Change `optVNAssertionPropCurStmt` to use GenTreeStmt.
Replace `GenTree* stmt = block->bbTreeList` with `GenTreeStmt* stmt = block->firstStmt()`.
Save results of `FirstNonPhiDef` as `GenTreeStmt`.

* Replace do-while with for loop.

* Change type inside VNAssertionPropVisitorInfo.

* Delete unused args fron `optVNConstantPropOnTree`.

* Update fields to be stmt.

Update optVNConstantPropCurStmt to use Stmt.
Change `lvDefStmt` to stmt.
Update LoopCloning structs.
Update `optDebugLogLoopCloning`.
Make `compCurStmt` a statement.
Update declaration name in `BuildNode`.

* Clean simple cpp files.

Clean valuenum.
Clean ssabuilder.
Clean simd.
Clean optcse.
Clean loopcloning.
Clean copyprop.
Clean optimizer part1.

* Start cleaning importer, morph, flowgraph, gentree.

* Continue clean functons.

Clean assertionprop.
Clean morph.
Clean gentree.
Clean flowgraph.
Clean compiler.
Clean rangecheck.
Clean indirectcalltransofrmer.
Clean others.

* Create some temp stmt.

* Delete unnecessary noway_assert and casts.

* Init `impStmtList` and `impLastStmt` in release.

* Response review 1.

1 files changed, 9 insertions, 12 deletions

diff --git a/src/jit/valuenum.cpp b/src/jit/valuenum.cpp
index 4280d872b0..88fbbcc3f0 100644
--- a/src/jit/valuenum.cpp
+++ b/src/jit/valuenum.cpp
@@ -5759,10 +5759,9 @@ void Compiler::fgValueNumber()
         for (BasicBlock* blk = fgFirstBB; blk != nullptr; blk = blk->bbNext)
         {
             // Now iterate over the block's statements, and their trees.
-            for (GenTree* stmts = blk->FirstNonPhiDef(); stmts != nullptr; stmts = stmts->gtNext)
+            for (GenTreeStmt* stmt = blk->FirstNonPhiDef(); stmt != nullptr; stmt = stmt->getNextStmt())
             {
-                assert(stmts->IsStatement());
-                for (GenTree* tree = stmts->gtStmt.gtStmtList; tree; tree = tree->gtNext)
+                for (GenTree* tree = stmt->gtStmtList; tree != nullptr; tree = tree->gtNext)
                 {
                     tree->gtVNPair.SetBoth(ValueNumStore::NoVN);
                 }
@@ -5925,12 +5924,12 @@ void Compiler::fgValueNumberBlock(BasicBlock* blk)
 
     // First: visit phi's.  If "newVNForPhis", give them new VN's.  If not,
     // first check to see if all phi args have the same value.
-    GenTree* firstNonPhi = blk->FirstNonPhiDef();
-    for (GenTree* phiDefs = blk->bbTreeList; phiDefs != firstNonPhi; phiDefs = phiDefs->gtNext)
+    GenTreeStmt* firstNonPhi = blk->FirstNonPhiDef();
+    for (GenTreeStmt* phiDefStmt = blk->firstStmt(); phiDefStmt != firstNonPhi; phiDefStmt = phiDefStmt->getNextStmt())
     {
         // TODO-Cleanup: It has been proposed that we should have an IsPhiDef predicate.  We would use it
         // in Block::FirstNonPhiDef as well.
-        GenTree* phiDef = phiDefs->gtStmt.gtStmtExpr;
+        GenTree* phiDef = phiDefStmt->gtStmtExpr;
         assert(phiDef->OperGet() == GT_ASG);
         GenTreeLclVarCommon* newSsaVar = phiDef->gtOp.gtOp1->AsLclVarCommon();
 
@@ -6133,21 +6132,19 @@ void Compiler::fgValueNumberBlock(BasicBlock* blk)
     }
 
     // Now iterate over the remaining statements, and their trees.
-    for (GenTree* stmt = firstNonPhi; stmt != nullptr; stmt = stmt->gtNext)
+    for (GenTreeStmt* stmt = firstNonPhi; stmt != nullptr; stmt = stmt->getNextStmt())
     {
-        assert(stmt->IsStatement());
-
 #ifdef DEBUG
         compCurStmtNum++;
         if (verbose)
         {
             printf("\n***** " FMT_BB ", stmt %d (before)\n", blk->bbNum, compCurStmtNum);
-            gtDispTree(stmt->gtStmt.gtStmtExpr);
+            gtDispTree(stmt->gtStmtExpr);
             printf("\n");
         }
 #endif
 
-        for (GenTree* tree = stmt->gtStmt.gtStmtList; tree; tree = tree->gtNext)
+        for (GenTree* tree = stmt->gtStmtList; tree != nullptr; tree = tree->gtNext)
         {
             fgValueNumberTree(tree);
         }
@@ -6156,7 +6153,7 @@ void Compiler::fgValueNumberBlock(BasicBlock* blk)
         if (verbose)
         {
             printf("\n***** " FMT_BB ", stmt %d (after)\n", blk->bbNum, compCurStmtNum);
-            gtDispTree(stmt->gtStmt.gtStmtExpr);
+            gtDispTree(stmt->gtStmtExpr);
             printf("\n");
             if (stmt->gtNext)
             {