i386: Don't emit pushf;pop for __builtin_ia32_readeflags_u* with unused lhs [PR104971]

i386: Don't emit pushf;pop for __builtin_ia32_readeflags_u* with unused lhs [PR104971]

__builtin_ia32_readeflags_u* aren't marked const or pure I think
intentionally, so that they aren't CSEd from different regions of a function
etc. because we don't and can't easily track all dependencies between
it and surrounding code (if somebody looks at the condition flags, it is
dependent on the vast majority of instructions).
But the builtin itself doesn't have any side-effects, so if we ignore the
result of the builtin, there is no point to emit anything.

There is a LRA bug that miscompiles the testcase which this patch makes
latent, which is certainly worth fixing too, but IMHO this change
(and maybe ix86_gimple_fold_builtin too which would fold it even earlier
when it looses lhs) is worth it as well.

2022-03-19  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/104971
	* config/i386/i386-expand.cc
	(ix86_expand_builtin) <case IX86_BUILTIN_READ_FLAGS>: If ignore,
	don't push/pop anything and just return const0_rtx.

	* gcc.target/i386/pr104971.c: New test.