c++: Don't shortcut TREE_CONSTANT vector type CONSTRUCTORs in cxx_eval_constant_expre...

c++: Don't shortcut TREE_CONSTANT vector type CONSTRUCTORs in cxx_eval_constant_expression [PR107295]

The excess precision support broke building skia (dependency of firefox)
on ia32 (it has something like the a constexpr variable), but as the other
cases show, it is actually a preexisting problem if one uses casts from
constants with wider floating point types.
The problem is that cxx_eval_constant_expression tries to short-cut
processing of TREE_CONSTANT CONSTRUCTORs if they satisfy
reduced_constant_expression_p - instead of calling cxx_eval_bare_aggregate
on them it just verifies flags and if they are TREE_CONSTANT even after
that, just fold.
Now, on the testcase we have a TREE_CONSTANT CONSTRUCTOR containing
TREE_CONSTANT NOP_EXPR of REAL_CST.  And, fold, which isn't recursive,
doesn't optimize that into VECTOR_CST, while later on we are only able
to optimize VECTOR_CST arithmetics, not arithmetics with vector
CONSTRUCTORs.
The following patch fixes that by rejecting CONSTRUCTORs with vector type
in reduced_constant_expression_p regardless of whether they have
CONSTRUCTOR_NO_CLEARING set or not, folding result in cxx_eval_bare_aggregate
even if nothing has changed but it wasn't non-constant and removing folding
from the TREE_CONSTANT reduced_constant_expression_p short-cut.

2022-10-21  Jakub Jelinek  <jakub@redhat.com>

	PR c++/107295
	* constexpr.cc (reduced_constant_expression_p) <case CONSTRUCTOR>:
	Return false for VECTOR_TYPE CONSTRUCTORs even without
	CONSTRUCTOR_NO_CLEARING set on them.
	(cxx_eval_bare_aggregate): If constant but !changed, fold before
	returning VECTOR_TYPE_P CONSTRUCTOR.
	(cxx_eval_constant_expression) <case CONSTRUCTOR>: Don't fold
	TREE_CONSTANT CONSTRUCTOR, just return it.

	* g++.dg/ext/vector42.C: New test.