[PATCH] [X86 DAG] Lower vselects with a constant condition as blend+imm if possible

[Filipe Cabecinhas]

Hi Nadav,

I'll separate the patch in three commits:
- Actually lower VSELECTs, possibly with no additional functionality
  - Optimize lowered VSELECTs when possible
- PerformSELECTCombine changes

  Filipe

http://reviews.llvm.org/D3752