[X86] Convert tbm_bextri_u32/tbm_bextri_u64 intrinsics TargetConstant argument...

[X86] Convert tbm_bextri_u32/tbm_bextri_u64 intrinsics TargetConstant argument to a regular Constant during lowering.

We reuse an ISD opcode here that can be reached from BMI that
doesn't require it to be an immediate. Our isel patterns to match
the TBM immediate form require a Constant and not a TargetConstant.

We were accidentally getting the Constant due to a quirk of
combineBEXTR calling SimplifyDemandedBits. The call to
SimplifyDemandedBits ended up constant folding the TargetConstant
to a regular Constant. But we should probably instead be asserting
if SimplifyDemandedBits on a TargetConstant so we shouldn't rely
on this behavior.

llvm-svn: 372373