[ARM] Remove unnecessary use of replaceSymbolicStrideSCEV (NFC).

When passing an empty strides map, there's nothing to replace for
replaceSymbolicStrideSCEV and it just returns the SCEV for Ptr. There
should be no need to call the function.

Reviewed By: SjoerdMeijer

Differential Revision: https://reviews.llvm.org/D109462