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

[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