Change null constant type to be TYP_REF for isinst nullcheck expansion

In the importer when inlining the fast expansion sequence for
isinst/castclass, the code was typing the constant 0 source of the
compare with TYP_I_IMPL instead of TYP_REF. Not only does this create
an inconsistently typed compare but it causes value-numbering of
the same constants with different types to not be equal. This blocks
later assertion propagation that propagates null-checks and prevents
many cases from being eliminated.

This change changes the type of the constant gtNode to be TYP_REF.
With this change there are many good diffs in the frameworks and
other assemblies.

Ran testing in desktop branch including SuperPMI asm diffs. Asm
diffs show hundreds of methods with improvements in framework
, Roslyn, and other internal assemblies. 1% code size reduction in
the methods that have changes. All diffs are elimination of redundant
null-checks.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/jit/importer.cpp b/src/jit/importer.cpp
index 652231085f..d55710400b 100644
--- a/src/jit/importer.cpp
+++ b/src/jit/importer.cpp
@@ -8178,7 +8178,7 @@ GenTreePtr Compiler::impCastClassOrIsInstToTree(GenTreePtr op1, GenTreePtr op2,
     //             op1Copy CNS_INT
     //                      null
     //                      
-    condNull = gtNewOperNode(GT_EQ, TYP_INT, gtClone(op1), gtNewIconNode(0, TYP_I_IMPL));  
+    condNull = gtNewOperNode(GT_EQ, TYP_INT, gtClone(op1), gtNewIconNode(0, TYP_REF));
 
     //
     // expand the true and false trees for the condMT