[X86] Fix a bug in the lowering of the mask of VSELECT.
X86 code to lower VSELECT messed a bit with the bits set in the mask of VSELECT
when it knows it can be lowered into BLEND. Indeed, only the high bits need to be
set for those and it optimizes those accordingly.
However, when the mask is a compile time constant, the lowering will be handled
by the generic optimizer and those modifications will generate bad code in the
generic optimizer.

This patch fixes that by preventing the optimization if the VSELECT will be
handled by the generic optimizer.

<rdar://problem/18675020>