rtl-ssa: Ensure global registers are live on exit

RTL-SSA mostly relies on DF for block-level register liveness
information, including artificial uses and defs at the beginning
and end of blocks.  But one case was missing.  DF does not add
artificial uses of global registers to the beginning or end
of a block.  Instead it marks them as used within every block
when computing LR and LIVE problems.

For RTL-SSA, global registers behave like memory, which in
turn behaves like gimple vops.  We need to ensure that they
are live on exit so that final definitions do not appear
to be unused.

Also, the previous live-on-exit handling only considered the exit
block itself.  It needs to consider non-local gotos as well, since
they jump directly to some code in a parent function and so do
not have a path to the exit block.

gcc/
	* rtl-ssa/blocks.cc (function_info::add_artificial_accesses): Force
	global registers to be live on exit.  Handle any block with zero
	successors like an exit block.

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