Rewrite the use optimizer to be less memory intensive and 50% faster.

Fixes PR28670

Summary:
Rewrite the use optimizer to be less memory intensive and 50% faster.
Fixes PR28670

The new use optimizer works like a standard SSA renaming pass, storing
all possible versions a MemorySSA use could get in a stack, and just
tracking indexes into the stack.
This uses much less memory than caching N^2 alias query results.
It's also a lot faster.

The current version defers phi node walking to the normal walker.

Reviewers: george.burgess.iv

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D23032

llvm-svn: 277480