[llvm] [GlobalISel] add KnownBits tracking for G_UMULH and G_SMULH (PR #158445)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Updated. Looks like the early exit reverted those changes to AMDGPU *.i64.ll tests

I guess then it's worthwhile to compute both halves anyway? 

https://github.com/llvm/llvm-project/pull/158445