Clean up in LowerStoreIndirCoalescing

[EgorBo]

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[EgorBo]

PTAL @jakobbotsch cc @dotnet/jit-contrib
A small clean up to allow more foldings I plan to try to land (e.g. load/copy merging, store-load).
As a bonus, it now allows to fold STOREIND_BLK, e.g.

*dst = default(Guid);
*dst = default(Guid);

hence, a few new diffs.