[nvptx] Don't skip atomic insns in nvptx_reorg_uniform_simt

[nvptx] Don't skip atomic insns in nvptx_reorg_uniform_simt

In nvptx_reorg_uniform_simt we have a loop:
...
  for (insn = get_insns (); insn; insn = next)
    {
      next = NEXT_INSN (insn);
      if (!(CALL_P (insn) && nvptx_call_insn_is_syscall_p (insn))
         && !(NONJUMP_INSN_P (insn)
              && GET_CODE (PATTERN (insn)) == PARALLEL
              && get_attr_atomic (insn)))
       continue;
...
that intends to handle syscalls and atomic insns.

However, this also silently skips the atomic insn nvptx_atomic_store, which
has GET_CODE (PATTERN (insn)) == SET.

This does not cause problems, because the nvptx_atomic_store actually maps
onto a "st" insn, and therefore is not atomic and doesn't need to be handled
by nvptx_reorg_uniform_simt.

Fix this by:
- explicitly setting nvptx_atomic_store's atomic attribute to false,
- rewriting the skip condition to make sure all insn
  with atomic attribute are handled, and
- asserting that all handled insns are PARALLEL.

Tested on nvptx.

gcc/ChangeLog:

2022-02-19  Tom de Vries  <tdevries@suse.de>

	* config/nvptx/nvptx.cc (nvptx_reorg_uniform_simt): Handle all
	insns with atomic attribute.  Assert that all handled insns are
	PARALLELs.
	* config/nvptx/nvptx.md (define_insn "nvptx_atomic_store<mode>"):
	Set atomic attribute to false.

gcc/testsuite/ChangeLog:

2022-02-19  Tom de Vries  <tdevries@suse.de>

	* gcc.target/nvptx/uniform-simt-3.c: New test.