[X86] Simplify lowerVectorShuffleAsBitMask to handle only integer VT's

Summary: This function is only called with integer VT arguments, so remove code that handles FP vectors.

Reviewers: RKSimon, craig.topper, delena, andreadb

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D26985

llvm-svn: 287743