[RISCV] Use the stack for MVT::f16 for fastcc when there are no other registers available

In D155502, we added code for the compiler to check GPR-s for f16
under zhinx. This commit adds code to hit the stack when we run out of
GPR-s.

With this patch and D155502, resolves #63922

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D155507