[nvptx] Handle asm insn in prevent_branch_around_nothing

[nvptx] Handle asm insn in prevent_branch_around_nothing

With GOMP_NVPTX_JIT=-00 and -mptx=3.1, I run into:
...
FAIL: libgomp.oacc-c/../libgomp.oacc-c-c++-common/acc_prof-version-1.c \
  -DACC_DEVICE_TYPE_nvidia=1 -DACC_MEM_SHARED=0 -foffload=nvptx-none  -O2 \
  execution test
...

The problem is that we're generating a diverging branch around nothing:
...
        {
                .reg.u32        %x;
                mov.u32 %x, %tid.x;
                setp.ne.u32     %r23, %x, 0;
        }
        @%r23   bra     $L2;
$L2:
...
which the driver JIT has problems with at -O0, so consequently we run into the
nvptx_uniform_warp_check.

Fix this by handling asm ("") and alike in prevent_branch_around_nothing.

Tested on x86_64 with nvptx accelerator.

gcc/ChangeLog:

2022-02-10  Tom de Vries  <tdevries@suse.de>

	PR target/104456
	* config/nvptx/nvptx.cc (prevent_branch_around_nothing): Handle asm
	insn.