[PR target/123911][RISC-V] Fix infinite recursion in riscv_legitimize_move

[PR target/123911][RISC-V] Fix infinite recursion in riscv_legitimize_move

I kept hoping I'd see a better solution, perhaps one where chunks of this
routine just go away, but that hasn't materialized.  So...

This patch avoids infinite recursion through riscv_legitimize_move.
Essentially we end up calling it recursively with arguments that are a nop-move
and those particular arguments trigger infinite recursion.

So this patch just recognizes and elides the nop move.   Bootstrapped on
riscv64-linux-gnu and regression tested on riscv{32,64}-elf with no
regressions.  Pushing to the trunk.

	PR target/123911
gcc/
	* config/riscv/riscv.cc (riscv_legitimize_move): Elide nop moves to
	avoid infinite recursion.

gcc/testsuite/
	* gcc.target/riscv/pr123911.c: New test.