ifcombine field merge: handle masks with sign extensions

ifcombine field merge: handle masks with sign extensions

When a loaded field is sign extended, masked and compared, we used to
drop from the mask the bits past the original field width, which is
not correct.

Take note of the fact that the mask covered copies of the sign bit,
before clipping it, and arrange to test the sign bit if we're
comparing with zero.  Punt in other cases.

If bits_test fail recoverably, try other ifcombine strategies.

for  gcc/ChangeLog

	* gimple-fold.cc (decode_field_reference): Add psignbit
	parameter.  Set it if the mask references sign-extending
	bits.
	(fold_truth_andor_for_ifcombine): Adjust calls with new
	variables.  Swap them along with other r?_* variables.  Handle
	extended sign bit compares with zero.
	* tree-ssa-ifcombine.cc (ifcombine_ifandif): If bits_test
	fails in a way that doesn't prevent other ifcombine strategies
	from passing, give them a try.

for  gcc/testsuite/ChangeLog

	* gcc.dg/field-merge-16.c: New.