X86 ISel: Basic support for variable-index vector permutations

Summary:
Try to lower a BUILD_VECTOR composed of extract-extract chains that can be
reasoned to be a permutation of a vector by indices in a non-constant vector.

We saw this pattern created by ISPC, which resolts to creating it due to the
requirement that shufflevector's mask operand be a *constant* vector.
I didn't check this but we could possibly use this pattern for lowering the X86 permute
C-instrinsics instead of llvm.x86 instrinsics.

This change can be followed by more improvements:
1. Handle vectors with undef elements.
2. Utilize pshufb and zero-mask-blending to support more effiecient
   construction of vectors with constant-0 elements.
3. Use smaller-element vectors of same width, and "interpolate" the indices,
   when no native operation available.

Reviewers: RKSimon, craig.topper

Reviewed By: RKSimon

Subscribers: chandlerc, DavidKreitzer

Differential Revision: https://reviews.llvm.org/D39126

llvm-svn: 317463