[X86] Use TargetConstant for condition code on X86ISD::SETCC/CMOV/BRCOND nodes.

This removes the need for ConvertToTarget opcodes in the isel table.
It's also consistent with the recent changes to use TargetConstant
for intrinsic nodes that always take immediates.

Differential Revision: https://reviews.llvm.org/D67902

llvm-svn: 372645