[PATCH] [X86] Look for scalar through one bitcast when lowering to VBROADCAST.

[Ahmed Bougacha]

Hi chandlerc, spatel,

Fixes PR23464: one way to use the broadcast intrinsics is something like:

    _mm256_broadcastw_epi16(_mm_cvtsi32_si128(*(int*)src));

We don't currently fold these, but we can look through one bitcast to find the broadcast scalar.

Note that I added a file specifically for these tests. I can add these to the various vector-shuffle-* files;  I don't have an argument/preference for either.

http://reviews.llvm.org/D10557

Files:
  lib/Target/X86/X86ISelLowering.cpp
  lib/Target/X86/X86InstrSSE.td
  test/CodeGen/X86/vector-shuffle-pbroadcast-load.ll

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