aarch64: Fix -mtrack-speculation for irreversible conditions [PR93341]

We can't yet represent the inverse of all conditions in rtl
(see g:865257c4), triggering an ICE in the pass
that handles -mtrack-speculation.  Since we don't expect these
insns to be optimised in any way, the easiest fix seemed to be
to add an insn that reverses the condition internally.

2020-01-23  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR target/93341
	* config/aarch64/aarch64.md (UNSPEC_SPECULATION_TRACKER_REV): New
	unspec.
	(speculation_tracker_rev): New pattern.
	* config/aarch64/aarch64-speculation.cc (aarch64_do_track_speculation):
	Use speculation_tracker_rev to track the inverse condition.

gcc/testsuite/
	PR target/93341
	* gcc.target/aarch64/pr93341.c: New test.