[DA] Remove base pointers from subscripts (NFCI) (#157083)
This patch removes base pointers from subscripts when delinearization
fails. Previously, in such cases, the pointer type SCEVs were used
instead of offset SCEVs derived from them.
For example, here is a portion of the debug output when analyzing
`strong0` in `test/Analysis/DependenceAnalysis/StrongSIV.ll`:
```
testing subscript 0, SIV
src = {(8 + %A),+,4}<nuw><%for.body>
dst = {(8 + %A),+,4}<nuw><%for.body>
Strong SIV test
Coeff = 4, i64
SrcConst = (8 + %A), ptr
DstConst = (8 + %A), ptr
Delta = 0, i64
UpperBound = (-1 + %n), i64
Distance = 0
Remainder = 0
```
As shown above, the `SrcConst` and `DstConst` are pointer values rather
than integer offsets. `%A` should be removed.
This change is necessary for #157086, since
`ScalarEvolution::willNotOverflow` expects integer type SCEVs as
arguments. This change alone alone should not affect the analysis
results.
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