[ifcombine] check and extend constants to compare with bitfields

Add logic to check and extend constants compared with bitfields, so
that fields are only compared with constants they could actually
equal.  This involves making sure the signedness doesn't change
between loads and conversions before shifts: we'd need to carry a lot
more data to deal with all the possibilities.


for  gcc/ChangeLog

	PR tree-optimization/118456
	* gimple-fold.cc (decode_field_reference): Punt if shifting
	after changing signedness.
	(fold_truth_andor_for_ifcombine): Check extension bits in
	constants before clipping.

for  gcc/testsuite/ChangeLog

	PR tree-optimization/118456
	* gcc.dg/field-merge-21.c: New.
	* gcc.dg/field-merge-22.c: New.