i386: Fix handling of SUBREGs in divv2sf3 [PR103842]

i386: Fix handling of SUBREGs in divv2sf3 [PR103842]

register_operand predicate allows not just REGs, but also SUBREGs of REGs,
and for the latter lowpart_subreg might FAIL when trying to create paradoxical
SUBREG in some cases.  For the input operand fixed by force_reg on it first,
for the output operand handled by always dividing into a fresh V4SFmode temporary
and emit_move_insn into the destination afterwards, that is also beneficial for
combine.

2021-12-28  Jakub Jelinek  <jakub@redhat.com>

	PR target/103842
	* config/i386/mmx.md (divv2sf3): Use force_reg on op1.  Always perform
	divv4sf3 into a pseudo and emit_move_insn into operands[0].

	* g++.dg/opt/pr103842.C: New test.