[WebAssembly] Make register stackification more conservative

Register stackification currently checks VNInfo for changes. Make that
more accurate by testing each intervening instruction for any other defs
to the same virtual register.

Patch by Jacob Gravelle

Differential Revision: https://reviews.llvm.org/D24942

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@282886 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed