[nvptx] Assert fork has at most one join in nvptx_find_par

With the test-case of PR90009 we run into a SIGSEGV in nvptx_neuter_pars,
because par->join_insn and par->join_block are NULL.

Detect this problem earlier, by adding an assert in nvptx_find_par that
asserts that a fork cannot be paired with more than one join.

Build and tested on x86_64 with nvptx accelerator enabled.

2019-06-12  Tom de Vries  <tdevries@suse.de>

	PR tree-optimization/90009
	* config/nvptx/nvptx.c (nvptx_find_par): Assert fork has at most join.

From-SVN: r272189

gcc/ChangeLog

+2019-06-12  Tom de Vries  <tdevries@suse.de>
+
+	PR tree-optimization/90009
+	* config/nvptx/nvptx.c (nvptx_find_par): Assert fork has at most join.
+
 2019-06-12  Martin Liska  <mliska@suse.cz>
 
 	* ggc-common.c (ggc_prune_overhead_list): Do not sanitize

gcc/config/nvptx/nvptx.c

@@ -3258,6 +3258,7 @@ nvptx_find_par (bb_insn_map_t *map, parallel *par, basic_block block)
 	    unsigned mask = UINTVAL (XVECEXP (PATTERN (end), 0, 0));
 
 	    gcc_assert (par->mask == mask);
+	    gcc_assert (par->join_block == NULL);
 	    par->join_block = block;
 	    par->join_insn = end;
 	    if (nvptx_needs_shared_bcast (mask))