target/104581 - compile-time regression in mode-switching

target/104581 - compile-time regression in mode-switching

The x86 backend piggy-backs on mode-switching for insertion of
vzeroupper.  A recent improvement there was implemented in a way
to walk possibly the whole basic-block for all DF reg def definitions
in its mode_needed hook which is called for each instruction in
a basic-block during mode-switching local analysis.

The following mostly reverts this improvement.  It needs to be
re-done in a way more consistent with a local dataflow which
probably means making targets aware of the state of the local
dataflow analysis.

2022-02-17  Richard Biener  <rguenther@suse.de>

	PR target/104581
	* config/i386/i386.cc (ix86_avx_u128_mode_source): Remove.
	(ix86_avx_u128_mode_needed): Return AVX_U128_DIRTY instead
	of calling ix86_avx_u128_mode_source which would eventually
	have returned AVX_U128_ANY in some very special case.

	* gcc.target/i386/pr101456-1.c: XFAIL.