i386: Fix up _mm256_vzeroupper() handling [PR99563]

i386: Fix up _mm256_vzeroupper() handling [PR99563]

My r10-6451-gb7b3378f91c0641f2ef4d88db22af62a571c9359 fix for
vzeroupper vs. ms ABI apparently broke the explicit vzeroupper handling
when the implicit vzeroupper handling is disabled.
The epilogue_completed splitter for vzeroupper now adds clobbers for all
registers which don't have explicit sets in the pattern and the sets are
added during vzeroupper pass.  Before my changes, for explicit user
vzeroupper, we just weren't modelling its effects at all, it was just
unspec that didn't tell that it clobbers the upper parts of all XMM < %xmm16
registers.  But now the splitter will even for those add clobbers and as
it has no sets, it will add clobbers for all registers, which means
we optimize away anything that lived across that vzeroupper.

The vzeroupper pass has two parts, one is the mode switching that computes
where to put the implicit vzeroupper calls and puts them there, and then
another that uses df to figure out what sets to add to all the vzeroupper.
The former part should be done only under the conditions we have in the
gate, but the latter as this PR shows needs to happen either if we perform
the implicit vzeroupper additions, or if there are (or could be) any
explicit vzeroupper instructions.  As that function does df_analyze and
walks the whole IL, I think it would be too expensive to run it always
whenever TARGET_AVX, so this patch remembers if we've expanded at least
one __builtin_ia32_vzeroupper in the function and runs that part of the
vzeroupper pass both when the old condition is true or when this new
flag is set.

2021-03-16  Jakub Jelinek  <jakub@redhat.com>

	PR target/99563
	* config/i386/i386.h (struct machine_function): Add
	has_explicit_vzeroupper bitfield.
	* config/i386/i386-expand.c (ix86_expand_builtin): Set
	cfun->machine->has_explicit_vzeroupper when expanding
	IX86_BUILTIN_VZEROUPPER.
	* config/i386/i386-features.c (rest_of_handle_insert_vzeroupper):
	Do the mode switching only when TARGET_VZEROUPPER, expensive
	optimizations turned on and not optimizing for size.
	(pass_insert_vzeroupper::gate): Enable even when
	cfun->machine->has_explicit_vzeroupper is set.

	* gcc.target/i386/avx-pr99563.c: New test.