[CGP] Make optimizeMemoryInst capable of handling multiple AddrModes

Currently optimizeMemoryInst requires that all of the AddrModes it sees are
identical. This patch makes it capable of tracking multiple AddrModes, so long
as they differ in at most one field.

This patch does nothing by itself, but later patches will make use of it to
insert or reuse phi or select instructions for the differing fields.

Differential Revision: https://reviews.llvm.org/D38278

llvm-svn: 314795