[AMDGPU] Narrow lshl from 64 to 32 bit if possible

[AMDGPU] Narrow lshl from 64 to 32 bit if possible

Turn expensive 64 bit shift into 32 bit if shift does not overflow int:
shl (ext x) => zext (shl x)

Differential Revision: https://reviews.llvm.org/D33367

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@303569 91177308-0d34-0410-b5e6-96231b3b80d8