[ifcombine] improve reverse checking and operand swapping

[ifcombine] improve reverse checking and operand swapping

Don't reject an ifcombine field-merging opportunity just because the
left-hand operands aren't both reversed, if the second compare needs
to be swapped for operands to match.

Also mention that reversep does NOT affect the turning of range tests
into bit tests.

for  gcc/ChangeLog

	* gimple-fold.cc (fold_truth_andor_for_ifcombine): Document
	reversep's absence of effects on range tests.  Don't reject
	reversep mismatches before trying compare swapping.