JIT: In morph, only call DefinesLocal on assignments (#22753)

When checking for local assertions to kill in morph, only call `DefinesLocal` on `GT_ASG` nodes.

Also, assert that we never see LIR style assignments.

Resolves #22747.

1 files changed, 10 insertions, 1 deletions

diff --git a/src/jit/morph.cpp b/src/jit/morph.cpp
index ff9320ab26..f3556ad001 100644
--- a/src/jit/morph.cpp
+++ b/src/jit/morph.cpp
@@ -15184,7 +15184,16 @@ void Compiler::fgMorphTreeDone(GenTree* tree,
     {
         /* Is this an assignment to a local variable */
         GenTreeLclVarCommon* lclVarTree = nullptr;
-        if (tree->DefinesLocal(this, &lclVarTree))
+
+        // The check below will miss LIR-style assignments.
+        //
+        // But we shouldn't be running local assertion prop on these,
+        // as local prop gets disabled when we run global prop.
+        assert(!tree->OperIs(GT_STORE_LCL_VAR, GT_STORE_LCL_FLD));
+
+        // DefinesLocal can return true for some BLK op uses, so
+        // check what gets assigned only when we're at an assignment.
+        if (tree->OperIs(GT_ASG) && tree->DefinesLocal(this, &lclVarTree))
         {
             unsigned lclNum = lclVarTree->gtLclNum;
             noway_assert(lclNum < lvaCount);