i386: Fix up ix86_convert_const_wide_int_to_broadcast [PR108599]

i386: Fix up ix86_convert_const_wide_int_to_broadcast [PR108599]

The following testcase is miscompiled.  The problem is that during
RTL DSE we see a V4DI register is being loaded { 16, 16, 0, 0 }
value and DSE mostly works in terms of scalar modes, so it calls
movoi to set an OImode REG to (const_wide_int 0x100000000000000010)
and ix86_convert_const_wide_int_to_broadcast thinks it can compute
that value by broadcasting DImode 0x10.  While it is true that
for TImode result the broadcast could be used, for OImode/XImode
it can't be, because all but the lowest 2 HOST_WIDE_INTs aren't
present (so are 0 or -1 depending on sign), not 0x10 in this case.
The function checks if the least significant HOST_WIDE_INT elt
of the CONST_WIDE_INT is broadcastable from QI/HI/SI/DImode and then
  /* Check if OP can be broadcasted from VAL.  */
  for (int i = 1; i < CONST_WIDE_INT_NUNITS (op); i++)
    if (val != CONST_WIDE_INT_ELT (op, i))
      return nullptr;
That is needed of course, but nothing checks that
CONST_WIDE_INT_NUNITS (op) isn't too small for the mode in question.
I think if op would be 0 or -1, it ought to be never CONST_WIDE_INT,
but CONST_INT and so we can just punt whenever the number of
CONST_WIDE_INT elts is not the expected one.

2023-01-31  Jakub Jelinek  <jakub@redhat.com>

	PR target/108599
	* config/i386/i386-expand.cc
	(ix86_convert_const_wide_int_to_broadcast): Return nullptr if
	CONST_WIDE_INT_NUNITS (op) times HOST_BITS_PER_WIDE_INT isn't
	equal to bitsize of mode.

	* gcc.target/i386/avx2-pr108599.c: New test.