[PATCH] [BUG][INDVAR] Fix for PR21014: wrong SCEV operands commuting for non-commutative instructions

[Zinovy Nis]

Hi atrick, hfinkel, joerg,

My commit rL216160 introduced a bug PR21014. 

IndVars widens code


```
for (i = ; i < ...; i++) arr[ CONST - i]
```

into 


```
for (i = ; i < ...; i++) arr[ i - CONST]
```

thus inverting index expression. The patch attached fixes it. 
Thanks to Jörg Sonnenberger for pointing.

http://reviews.llvm.org/D5576

Files:
  lib/Transforms/Scalar/IndVarSimplify.cpp
  test/Transforms/IndVarSimplify/2011-09-10-widen-nsw.ll
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