[nvptx, PR85204] Fix neutering of bb with only cond jump

[Tom de Vries]

Hi,

When compiling the test-case in the patch, the following ptx code is 
generated:
...
$L4:
   @ %r91 bra.uni $L24;
   selp.u32 %r95,1,0,%r80;
   st.shared.u32 [__worker_bcast],%r95;
  $L25:
  $L24:
   @ %r92 bra $L25;
...

There's an eternal loop starting at the last insn, and unsurprisingly 
the test-case hangs.

The last insn is a vector neutering branch, which should have been 
inserted after the worker neutering branch (the first insn).

In other words, we want:
...
  $L4:
    @ %r91 bra.uni $L24;
+  @ %r92 bra $L25;
    selp.u32 %r95,1,0,%r80;
    st.shared.u32 [__worker_bcast],%r95;
   $L25:
   $L24:
-  @ %r92 bra $L25;
...

This minimal stage4 patch fixes this problem. [ I filed a PR85223 
"[nvptx] nvptx_single needs rewrite" for a stage1 rewrite of nvptx_single. ]

Build x86_64 with nvptx accelerator, and tested libgomp.

Committed to stage4 trunk.

Thanks,
- Tom
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