[X86][InstCombine] Add some simplifications for BZHI intrinsics

[X86][InstCombine] Add some simplifications for BZHI intrinsics

This intrinsic clears the upper bits starting at a specified index. If the index is a constant we can do some simplifications.

This could be in InstSimplify, but we don't handle any target specific intrinsics there today.

Differential Revision: https://reviews.llvm.org/D36069

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@309604 91177308-0d34-0410-b5e6-96231b3b80d8