[X86][InstCombine] Add basic simplification support for BEXTR/BEXTRI intrinsics.

This patch adds simplification support for the BEXTR/BEXTRI intrinsics to match gcc. This only supports cases that fold to 0 or can be fully constant folded. Theoretically we could support converting to AND if the shift part is unused or to only a shift if the mask doesn't modify any bits after an equivalent shl. gcc doesn't do these transformations either.

I put this in InstCombine, but it could be done in InstSimplify. It would be the first target specific intrinsic in InstSimplify.

Differential Revision: https://reviews.llvm.org/D36063

llvm-svn: 309603