i386: Fix up ix86_expand_vector_init_general [PR105123]

The following testcase is miscompiled on ia32.
The problem is that at -O0 we end up with:
  vector(4) short unsigned int _1;
  short unsigned int u.0_3;
...
  _1 = {u.0_3, u.0_3, u.0_3, u.0_3};
statement (dead) which is wrongly expanded.
elt is (subreg:HI (reg:SI 83 [ u.0_3 ]) 0), tmp_mode SImode,
so after convert_mode we start with word (reg:SI 83 [ u.0_3 ]).
The intent is to manually broadcast that value to 2 SImode parts,
but because we pass word as target to expand_simple_binop, it will
overwrite (reg:SI 83 [ u.0_3 ]) and we end up with 0:
   10: {r83:SI=r83:SI<<0x10;clobber flags:CC;}
   11: {r83:SI=r83:SI|r83:SI;clobber flags:CC;}
   12: {r83:SI=r83:SI<<0x10;clobber flags:CC;}
   13: {r83:SI=r83:SI|r83:SI;clobber flags:CC;}
   14: clobber r110:V4HI
   15: r110:V4HI#0=r83:SI
   16: r110:V4HI#4=r83:SI
as the two ors do nothing and two shifts each by 16 left shift it all
away.
The following patch fixes that by using NULL_RTX target, so we expand it as
   10: {r110:SI=r83:SI<<0x10;clobber flags:CC;}
   11: {r111:SI=r110:SI|r83:SI;clobber flags:CC;}
   12: {r112:SI=r83:SI<<0x10;clobber flags:CC;}
   13: {r113:SI=r112:SI|r83:SI;clobber flags:CC;}
   14: clobber r114:V4HI
   15: r114:V4HI#0=r111:SI
   16: r114:V4HI#4=r113:SI
instead.

Another possibility would be to pass NULL_RTX only when word == elt
and word otherwise, where word would necessarily be a pseudo from the first
shift after passing NULL_RTX there once or pass NULL_RTX for the shift and
word for ior.

2022-04-03  Jakub Jelinek  <jakub@redhat.com>

	PR target/105123
	* config/i386/i386-expand.cc (ix86_expand_vector_init_general): Avoid
	using word as target for expand_simple_binop when doing ASHIFT and
	IOR.

	* gcc.target/i386/pr105123.c: New test.