i386: Fix ICEs due to simplify_gen_subreg returning NULL [PR99025]

i386: Fix ICEs due to simplify_gen_subreg returning NULL [PR99025]

In these patterns, we call simplify_gen_subreg on the input operand
to create paradoxical subregs that have 2x, 4x or 8x elements as the input
operand.  That works fine if the input operand is a REG, but when it is a
SUBREG, RTL doesn't allow SUBREG of SUBREG and so relies on simplify_subreg
actually simplifying it.  And e.g. if the input operand is a SUBREG that
changes the element mode (floating vs. non-floating) and then combined with
a paradoxical subreg (i.e. different size) this can easily fail, then
simplify_gen_subreg returns NULL but we still use it in instructions.

Fixed by forcing the operands into REG.

2021-02-10  Jakub Jelinek  <jakub@redhat.com>

	PR target/99025
	* config/i386/sse.md (fix<fixunssuffix>_truncv2sfv2di2,
	<insn>v8qiv8hi2, <insn>v8qiv8si2, <insn>v4qiv4si2, <insn>v4hiv4si2,
	<insn>v8qiv8di2, <insn>v4qiv4di2, <insn>v2qiv2di2, <insn>v4hiv4di2,
	<insn>v2hiv2di2, <insn>v2siv2di2): Force operands[1] into REG before
	calling simplify_gen_subreg on it.

	* gcc.target/i386/pr99025.c: New test.