[GlobalISel] Fix store merging incorrectly classifying an unknown index expr as 0...

[GlobalISel] Fix store merging incorrectly classifying an unknown index expr as 0. (#90375)

During analysis, we incorrectly leave the offset part of an address info
struct
as zero, when in actual fact we failed to decompose it into base +
offset.
This results in incorrectly assuming that the address is adjacent to
another store
addr. To fix this we wrap the offset in an optional<> so we can
distinguish between
real zero and unknown.

Fixes issue #90242

(cherry picked from commit 19f4d68252b70c81ebb1686a5a31069eda5373de)