[CodeGen] Re-apply r234809 (concat of scalars), with an x86_mmx fix.

The only type that isn't an integer, isn't floating point, and isn't
a vector; ladies and gentlemen, the gift that keeps on giving: x86_mmx!

Fixes PR23246.

Original message (reverted in r235062):
[CodeGen] Combine concat_vectors of scalars into build_vector.

Combine something like:
  (v8i8 concat_vectors (v2i8 bitcast (i16)) x4)
into:
  (v8i8 (bitcast (v4i16 BUILD_VECTOR (i16) x4)))

If any of the scalars are floating point, use that throughout.

Differential Revision: http://reviews.llvm.org/D8948

llvm-svn: 235072