[nvptx] Handle nop in prevent_branch_around_nothing

When running libgomp test-case reduction-7.c on an nvptx accelerator
(T400, driver version 470.86) and GOMP_NVPTX_JIT=-O0, I run into:
...
reduction-7.exe:reduction-7.c:312: v_p_2: \
  Assertion `out[j * 32 + i] == (i + j) * 2' failed.
FAIL: libgomp.oacc-c/../libgomp.oacc-c-c++-common/reduction-7.c \
  -DACC_DEVICE_TYPE_nvidia=1 -DACC_MEM_SHARED=0 -foffload=nvptx-none \
  -O0  execution test
...

During investigation I found ptx code like this:
...
@ %r163 bra $L262;
$L262:
...

There's a known problem with executing this type of code, and a workaround is
in place to address this: prevent_branch_around_nothing.  The workaround does
not trigger though because it doesn't handle the nop insn.

Fix this by handling the nop insn in prevent_branch_around_nothing.

Tested libgomp on x86_64 with nvptx accelerator.

gcc/ChangeLog:

2022-01-27  Tom de Vries  <tdevries@suse.de>

	PR target/100428
	* config/nvptx/nvptx.cc (prevent_branch_around_nothing): Handle nop
	insn.

gcc/config/nvptx/nvptx.cc

@@ -5103,6 +5103,7 @@ prevent_branch_around_nothing (void)
 	    case CODE_FOR_nvptx_forked:
 	    case CODE_FOR_nvptx_joining:
 	    case CODE_FOR_nvptx_join:
+	    case CODE_FOR_nop:
 	      continue;
 	    default:
 	      seen_label = NULL;