[ifcombine] check for more zero-extension cases [PR118572]

When comparing a signed narrow variable with a wider constant that has
the bit corresponding to the variable's sign bit set, we would check
that the constant is a sign-extension from that sign bit, and conclude
that the compare fails if it isn't.

When the signed variable is masked without getting the [lr]l_signbit
variable set, or when the sign bit itself is masked out, we know the
sign-extension bits from the extended variable are going to be zero,
so the constant will only compare equal if it is a zero- rather than
sign-extension from the narrow variable's precision, therefore, check
that it satisfies this property, and yield a false compare result
otherwise.


for  gcc/ChangeLog

	PR tree-optimization/118572
	* gimple-fold.cc (fold_truth_andor_for_ifcombine): Compare as
	unsigned the variables whose extension bits are masked out.

for  gcc/testsuite/ChangeLog

	PR tree-optimization/118572
	* gcc.dg/field-merge-24.c: New.