[X86] Use getZeroVector and remove an unnecessary creation of an APInt before...

[X86] Use getZeroVector and remove an unnecessary creation of an APInt before calling getConstant. NFCI

The getConstant function can take care of creating the APInt internally.

getZeroVector will take care of using the correct type for the build vector to avoid re-lowering.

The test change here is because execution domain constraints apparently pass through undef inputs of a zeroing xor. So the different ordering of register allocation here caused the dependency to change.

llvm-svn: 319725