[SPARC] Mark the %g0 register as constant & use it to materialize zeros

Materialize zeros by copying from %g0, which is now marked as constant.

This makes it possible for some common operations (like integer negation) to be
performed in fewer instructions.

This continues @arichardson's patch at D132561.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D138887