[nvptx] Add uniform_warp_check insn

On a GT 1030, with driver version 470.94 and -mptx=3.1 I run into:
...
FAIL: libgomp.oacc-c/../libgomp.oacc-c-c++-common/parallel-dims.c \
  -DACC_DEVICE_TYPE_nvidia=1 -DACC_MEM_SHARED=0 -foffload=nvptx-none \
  -O2 execution test
...
which minimizes to the same test-case as listed in commit "[nvptx]
Update default ptx isa to 6.3".

The problem is again that the first diverging branch is not handled as such in
SASS, which causes problems with a subsequent shfl insn, but given that we
have -mptx=3.1 we can't use the bar.warp.sync insn.

Given that the default is now -mptx=6.3, and consequently -mptx=3.1 is of a
lesser importance, implement the next best thing: abort when detecting
non-convergence using this insn:
...
  { .reg.b32 act;
    vote.ballot.b32 act,1;
    .reg.pred uni;
    setp.eq.b32 uni,act,0xffffffff;
    @ !uni trap;
    @ !uni exit;
  }
...

Interestingly, the effect of this is that rather than aborting, the test-case
now passes.

Tested on x86_64 with nvptx accelerator.

gcc/ChangeLog:

2022-01-31  Tom de Vries  <tdevries@suse.de>

	* config/nvptx/nvptx.cc (nvptx_single): Use nvptx_uniform_warp_check.
	* config/nvptx/nvptx.md (define_c_enum "unspecv"): Add
	UNSPECV_UNIFORM_WARP_CHECK.
	(define_insn "nvptx_uniform_warp_check"): New define_insn.

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