PR18102, PATCH + test: wrong code on x86_64-linux-gnu (affecting trunk and 3.3)

[Stepan Dyatkovskiy]

Hi all.

I think, trouble was in DAGCombiner::MergeConsequtiveStores.

Consider, how it works for next example:

store i8 1, a[0]
store i8 2, a[1]
store i8 3, a[1]   ; a[1] again.
return   ; DAG starts here

1. Method will collect all the 3 stores.
2. It sorts them by distance from the base pointer (farthest with 
highest index).
3. It takes first consecutive non-overlapping stores and (if possible) 
replaces them with a single store instruction.

The point is, we can't determine here which 'store' instruction
would be the second after sorting ('store 2' or 'store 3').
It happens that 'store 3' would be the second, and 'store 2' would be 
the third.

So after merging we have the next result:

store i16 (1 | 3 << 8), base   ; is a[0] but bit-casted to i16
store i8 2, a[1]

So actually we swapped 'store 3' and 'store 2' and got wrong contents in 
a[1].

Fix: In sort routine just also take into account mem-op sequence number.

-Stepan
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