Fix PR77881: combine improvement

[Andreas Schwab]

On Nov 14 2016, Michael Matz <matz@suse.de> wrote:

> 	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.

This breaks gcc.c-torture/execute/cbrt.c execution test on aarch64.

Andreas.

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Andreas Schwab, SUSE Labs, schwab@suse.de
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