[RISC-V][PR target/121983] Fix unprotected REGNO invocation

[RISC-V][PR target/121983] Fix unprotected REGNO invocation

So this was a naked REGNO usage, which of course blows up if RTL checking is
enabled and something else sneaks in (SUBREG).   Thankfully it never results in
incorrect code, though I could theorize it could cause a bootstrap comparison
failure in the "right" circumstances.

Bootstrapped & regression tested on the Pioneer.  Also regression tested on
riscv64-elf and riscv32-elf.   I'll push to the trunk once pre-commit CI is
done.

jeff

	PR target/121983
gcc/
	* config/riscv/riscv.cc (riscv_macro_fusion_pair_p): Make sure
	object is a REG before asking for its REGNO.  Fix a trivial
	whitespace nit.

gcc/testsuite/
	* gcc.target/riscv/pr121983.c: New test.