Set global epilogue_completed when skipping pro_and_epilogue pass

When compiling RTL functions marked to start at a pass after the reload
pass, `skip_pass` is used to mark the reload pass as having completed
since many patterns use the `reload_completed` variable to determine
whether to run or not.

Here we do the same for the `epilogue_completed` variable and the
pro_and_epilogue pass.

Also include a testcase that relies on the availability of a
define_split in the aarch64 backend that is conditioned on this
`epilogue_completed` variable.

regtest done on native aarch64
regtest done on native x64_86

gcc/ChangeLog:

2019-11-15  Matthew Malcomson  <matthew.malcomson@arm.com>

	* passes.c (skip_pass): Set epilogue_completed if skipping the
	pro_and_epilogue pass.

gcc/testsuite/ChangeLog:

2019-11-15  Matthew Malcomson  <matthew.malcomson@arm.com>

	* gcc.dg/rtl/aarch64/test-epilogue-set.c: New test.

From-SVN: r278285