isel: Small cleanup of duplicating comparisons

isel: Small cleanup of duplicating comparisons

This is a small cleanup of the duplicating comparison code.
There is code generation difference but only for -O0 and -fno-tree-ter
(both of which will be fixed in a later patch).
The difference is instead of skipping the first use if the
comparison uses are only in cond_expr we skip the last use.
Also we go through the uses list in the opposite order now too.

The cleanups are the following:
* Don't call has_single_use as we will do the loop anyways
* Change the order of the checks slightly, it is better
  to check for cond_expr earlier
* Use cond_exprs as a stack and pop from it.
  Skipping the top if the use is only from cond_expr.

Bootstrapped and tested on x86_64-linux-gnu with no regressions.

gcc/ChangeLog:

	* gimple-isel.cc (duplicate_comparison): Rename to ...
	(maybe_duplicate_comparison): This. Add check for use here
	rather than in its caller.
	(pass_gimple_isel::execute): Don't check how many uses the
	comparison had and call maybe_duplicate_comparison instead of
	duplicate_comparison.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>