[X86] Simplify some code in LowerBUILD_VECTORvXi1. NFCI

The case were Immediate is 0 and HasConstElts is true should never
happen since that would mean the constant elts were all zero. But
we check for all zero build vector earlier. So just use HasConstElts
and blindly take Immediate without checking if its 0.

Move the code that bitcasts and extract the immediate into the
the HasConstElts case since the other code just creates an undef
with the right type. No casting needed.

llvm-svn: 372153