[MergeICmps] Don't merge icmps derived from pointers with addressspaces

IIUC we can't emit `memcmp` between pointers in addressspaces,
doing so will trigger an assertion since the signature of the memcmp
will not match it's arguments (https://bugs.llvm.org/show_bug.cgi?id=48661).

This PR disables the attempt to merge icmps,
when the pointer is in an addressspace.

Reviewed By: #julialang, vtjnash

Differential Revision: https://reviews.llvm.org/D94813

(cherry picked from commit 4cacb5ca)