ifcombine field merge: stricten loads tests, swap compare to match

ifcombine field merge: stricten loads tests, swap compare to match

ACATS-4 ca11d02 exposed an error in the logic for recognizing and
identifying the inner object in decode_field_ref: a view-converting
load, inserted in a previous successful field merging operation, was
recognized by gimple_convert_def_p within decode_field_reference, and
as a result we took its operand as the expression, and failed to take
note of the load location.

Without that load, we couldn't compare vuses, and then we ended up
inserting a wider load before relevant parts of the object were
initialized.

This patch makes gimple_convert_def_p recognize loads only when
requested, and requires that either both or neither parts of a
potentially merged operand have associated loads.

As a bonus, it enables additional optimizations by swapping the
operands of the second compare when that makes left-hand operands
of both compares match.

for  gcc/ChangeLog

	* gimple-fold.cc (gimple_convert_def_p): Reject load stmts
	unless requested.
	(decode_field_reference): Accept a converting load at the last
	conversion matcher, subsuming the load identification.
	(fold_truth_andor_for_ifcombine): Refuse to merge operands
	when only one of them has an associated load stmt.  Swap
	operands of one of the compares if that helps them match.

for  gcc/testsuite/ChangeLog

	* gcc.dg/field-merge-13.c: New.