[nvptx] Use bit-bucket operand for atom insns

[nvptx] Use bit-bucket operand for atom insns

For an atomic fetch operation that doesn't use the result:
...
  __atomic_fetch_add (p64, v64, MEMMODEL_RELAXED);
...
we currently emit:
...
  atom.add.u64 %r26, [%r25], %r27;
...

Detect the REG_UNUSED reg-note for %r26, and emit instead:
...
  atom.add.u64 _, [%r25], %r27;
...

Likewise for all atom insns.

Tested on nvptx.

gcc/ChangeLog:

2022-03-07  Tom de Vries  <tdevries@suse.de>

	PR target/104815
	* config/nvptx/nvptx.cc (nvptx_print_operand): Handle 'x' operand
	modifier.
	* config/nvptx/nvptx.md: Use %x0 destination operand in atom insns.

gcc/testsuite/ChangeLog:

2022-03-07  Tom de Vries  <tdevries@suse.de>

	PR target/104815
	* gcc.target/nvptx/atomic-bit-bucket-dest.c: New test.