[X86] Improve EmitLoweredSelect for contiguous CMOV pseudo instructions.

This change improves EmitLoweredSelect() so that multiple contiguous CMOV pseudo
instructions with the same (or exactly opposite) conditions get lowered using a single
new basic-block. This eliminates unnecessary extra basic-blocks (and CFG merge points)
when contiguous CMOVs are being lowered.

Patch by: kevin.b.smith@intel.com
Differential Revision: http://reviews.llvm.org/D11428

llvm-svn: 244202