Use broadcasts to optimize overall size when loading constant splat vectors (x86-64 with AVX or AVX2).

We generate broadcast instructions on CPUs with AVX2 to load some constant splat vectors.
This patch should preserve all existing behavior with regular optimization levels,
but also use splats whenever possible when optimizing for *size* on any CPU with AVX or AVX2.

The tradeoff is up to 5 extra instruction bytes for the broadcast instruction to save
at least 8 bytes (up to 31 bytes) of constant pool data.

Differential Revision: http://reviews.llvm.org/D5347