expmed: Always use QImode for init_expmed set_zero_cost [PR119785]

expmed: Always use QImode for init_expmed set_zero_cost [PR119785]

This is a regression on some targets introduced I believe by r6-2055
which added mode argument to set_src_cost.

The problem here is that in the first iteration, mode is always QImode
and we get as -Os zero cost set_src_cost (const0_rtx, QImode, false).
But then we use the mode variable for iterating over int, partial int
and vector int modes, so for the second iteration we call set_src_cost
with mode which is at that time (machine_mode) (MAX_MODE_VECTOR_INT + 1).

In the x86 case that happens to be V2HFmode and we don't crash (and
compute the same 0 cost as we would for QImode).
But e.g. in the SPARC case (machine_mode) (MAX_MODE_VECTOR_INT + 1) is
MAX_MACHINE_MODE and that does all kinds of weird things especially
when doing ubsan bootstrap.

Fixed by always using QImode.

2025-04-14  Jakub Jelinek  <jakub@redhat.com>

	PR rtl-optimization/119785
	* expmed.cc (init_expmed): Always pass QImode rather than mode to
	set_src_cost passed to set_zero_cost.