[PATCH][store-merging] Use store order as tie-breaker in sort_by_bitpos

[Kyrill Tkachov]

Hi all,

As Alexander pointed out in the thread starting at [1] the 
sort_by_bitpos sorting function
was behaving badly when we had multiple stores at the same position.  He 
fixed that (thanks!)
but we can do better by not returning zero when the bitpositions are 
equal but by falling back
to comparing the order the stores appear in, which is guaranteed to be 
unique (barring other
bugs elsewhere).

This patch does that.

Bootstrapped and tested on aarch64-none-linux-gnu.

Ok for trunk?

Thanks,
Kyrill

[1] https://gcc.gnu.org/ml/gcc-patches/2017-07/msg00895.html

2017-09-13  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

     * gimple-ssa-store-merging.c (sort_by_bitpos): Compare store order
     when bitposition is the same.
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