[X86][MMX] Improve handling of 64-bit MMX constants

64-bit MMX constant generation usually ends up lowering into SSE instructions before being spilled/reloaded as a MMX type.

This patch bitcasts the constant to a double value to allow correct loading directly to the MMX register.

I've added MMX constant asm comment support to improve testing, it's better to always print the double values as hex constants as MMX is mainly an integer unit (and even with 3DNow! its just floats).

Differential Revision: https://reviews.llvm.org/D43616

llvm-svn: 326497