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

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

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

We don't currently fold this, but now that we use native IR for
the intrinsics (r245605), we can look through one bitcast to find
the broadcast scalar.

Differential Revision: http://reviews.llvm.org/D10557

llvm-svn: 245613