[MemorySSA] Split PtrIntPair as this fails on win/arm.

Summary: Split PtrIntPair into Instruction and OptionalAlias<Result>. The latter needs 3 bits, which appear unavailable on certain archs.

Subscribers: sanjoy, jlebar, Prazek, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D44268

llvm-svn: 327046