add masked-epilogue tuning

The following adds a x86 tuning to enable the use of AVX512 masked
epilogues in cases we heuristically determine it to be not detrimental
by high chance.  Basically problematic cases are when there are
data streams that are both stored and loaded from and an outer loop
could end up executing only the inner loop masked epilogue and with
unlucky data stream advacement from the outer loop end up needing
to forward from masked stores to masked loads.  This isn't very
well handled, esp. for the case where unmasked operations would
not need to forward at all - that is, when forwarding completely
from the masked out portion of the store (like the AVX upper half
to the AVX lower half of a load).  There's also the case where
the number of iterations is known at compile time, only with
cost comparing we'd consider a non-masked epilog - as we are not
doing that we have to add heuristics to avoid masking when a
single vector epilog iteration would cover all scalar iterations
left (this is exercised by gcc.target/i386/pr110310.c).

SPEC CPU 2017 shows 3% text size savings over not using masked
epilogues with performance impact in the noise.  Masking all vector
epilogues gets that to 4% text size savings with some major
runtime regressions in 503.bwaves_r and 527.cam4_r
(measured on a Zen4 system), we're leaving a 5% improvement
for 549.fotonik3d_r unrealized with the implemented heuristic.

With the heuristics we turn 22513 vector epilogues + up to 12305 scalar
epilogues into 12305 masked vector epilogues of which 574 are for
AVX vector sizes, 79 for SSE vector sizes and the rest for AVX512.
When masking all epilogues we get 14567 of them from
29467 vector + up to 14567 scalar epilogues, so the heuristics disable
an additional 20% of masked epilogues.

	* config/i386/x86-tune.def (X86_TUNE_AVX512_MASKED_EPILOGUES):
	New tunable, default on for m_ZNVER4 and m_ZNVER5.
	* config/i386/i386.cc (ix86_vector_costs::finish_cost): With
	X86_TUNE_AVX512_MASKED_EPILOGUES and when the main loop
	had a vectorization factor > 2 use a masked epilogue when
	possible and when not obviously problematic.

	* gcc.target/i386/vect-mask-epilogue-1.c: New testcase.
	* gcc.target/i386/vect-mask-epilogue-2.c: Likewise.
	* gcc.target/i386/vect-epilogues-3.c: Adjust.

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