[omp, simt] Fix expand_GOMP_SIMT_*

[omp, simt] Fix expand_GOMP_SIMT_*

When running the test-case included in this patch using an
nvptx accelerator, it fails in execution.

The problem is that the expansion of GOMP_SIMT_XCHG_BFLY is optimized away
during pass_jump as "trivially dead insns".

This is caused by this code in expand_GOMP_SIMT_XCHG_BFLY:
...
  class expand_operand ops[3];
  create_output_operand (&ops[0], target, mode);
  ...
  expand_insn (targetm.code_for_omp_simt_xchg_bfly, 3, ops);
...
which doesn't guarantee that target is assigned to by the expanded insn.

F.i., if target is:
...
(gdb) call debug_rtx ( target )
(subreg/s/u:QI (reg:SI 40 [ _61 ]) 0)
...
then after expand_insn, we have:
...
(gdb) call debug_rtx ( ops[0].value )
(reg:QI 57)
...

See commit 3af3bec2e4d "internal-fn: Avoid dropping the lhs of some
calls [PR94941]" for a similar problem.

Fix this in the same way, by adding:
...
  if (!rtx_equal_p (target, ops[0].value))
    emit_move_insn (target, ops[0].value);
...
where applicable in the expand_GOMP_SIMT_* functions.

Tested libgomp on x86_64 with nvptx accelerator.

gcc/ChangeLog:

2021-04-28  Tom de Vries  <tdevries@suse.de>

	PR target/100232
	* internal-fn.c (expand_GOMP_SIMT_ENTER_ALLOC)
	(expand_GOMP_SIMT_LAST_LANE, expand_GOMP_SIMT_ORDERED_PRED)
	(expand_GOMP_SIMT_VOTE_ANY, expand_GOMP_SIMT_XCHG_BFLY)
	(expand_GOMP_SIMT_XCHG_IDX): Ensure target is assigned to.