match.pd: Avoid simplification into invalid BIT_FIELD_REFs [PR112673]

match.pd: Avoid simplification into invalid BIT_FIELD_REFs [PR112673]

The following testcase is lowered by the bitint lowering pass, then
vectorizer vectorizes one of the loops in it, so we have
  vect__18.6_34 = VIEW_CONVERT_EXPR<vector(4) unsigned long>(x_35(D));
  _8 = BIT_FIELD_REF <vect__18.6_34, 64, 0>;
...
  _18 = BIT_FIELD_REF <vect__18.6_34, 64, 64>;
etc. where x_35(D) is _BitInt(256) argument.  That is valid BIT_FIELD_REF,
the first argument is a vector and it extracts the vector elements from it.
Then comes forwprop4 and simplifies that using match.pd into
  _8 = (unsigned long) x_35(D);
...
  _18 = BIT_FIELD_REF <x_35(D), 64, 64>;
and tree-cfg verification ICEs on the latter (though, even the first cast
is kind of undesirable after bitint lowering, we want large/huge bitints
lowered).  The ICE is because if BIT_FIELD_REFs first argument has
INTEGRAL_TYPE_P, we require type_has_mode_precision_p, but that is not the
case of _BitInt(256), it has BLKmode.

The following patch fixes it by doing the BIT_FIELD_REF with VCE to
BIT_FIELD_REF simplification only if the result is valid.

2023-11-24  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/112673
	* match.pd (bit_field_ref (vce @0) -> bit_field_ref @0): Only simplify
	if either @0 doesn't have scalar integral type or if it has mode
	precision.

	* gcc.dg/pr112673.c: New test.