Introduce a basic MemorySSA updater, that supports insertDef,

Description

Introduce a basic MemorySSA updater, that supports insertDef,
insertUse, moveBefore and moveAfter operations.

Summary:
This creates a basic MemorySSA updater that handles arbitrary
insertion of uses and defs into MemorySSA, as well as arbitrary
movement around the CFG. It replaces the current splice API.

It can be made to handle arbitrary control flow changes.
Currently, it uses the same updater algorithm from D28934.

The main difference is because MemorySSA is single variable, we have
the complete def and use list, and don't need anyone to give it to us
as part of the API. We also have to rename stores below us in some
cases.

If we go that direction in that patch, i will merge all the updater
implementations (using an updater_traits or something to provide the
get* functions we use, called read*/write* in that patch).

Sadly, the current SSAUpdater algorithm is way too slow to use for
what we are doing here.

I have updated the tests we have to basically build memoryssa
incrementally using the updater api, and make sure it still comes out
the same.

Reviewers: george.burgess.iv

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29047