[LoongArch] Implement the convertSelectOfConstantsToMath hook

Normally, the ISD::SELECT is lowered to maskeqz + masknez. This is
inefficient when operands are constants that can be computed easily.
To avoid that, override TargetLowering::convertSelectOfConstantsToMath.

Differential Revision: https://reviews.llvm.org/D143621