re PR target/77881 (Non-optimal signed comparison on x86_64 since r146817)

	PR missed-optimization/77881
	* combine.c (simplify_comparison): Remove useless subregs
	also inside the loop, not just after it.
	(make_compound_operation): Recognize some subregs as being
	masking as well.

testsuite/
	* gcc.target/i386/pr77881.c: New test.

From-SVN: r242414