Make promoteLoopAccessesToScalars independent of AliasSet [NFC]

Summary:
The current promoteLoopAccessesToScalars method receives an AliasSet, but
the information used is in fact a list of Value*, known to must alias.
Create the list ahead of time to make this method independent of the AliasSet class.

While there is no functionality change, this adds overhead for creating
a set of Value*, when promotion would normally exit earlier.
This is meant to be as a first refactoring step in order to start replacing
AliasSetTracker with MemorySSA.
And while the end goal is to redesign LICM, the first few steps will focus on
adding MemorySSA as an alternative to the AliasSetTracker using most of the
existing functionality.

Reviewers: mkuper, danielcdh, dberlin

Subscribers: sanjoy, chandlerc, gberry, davide, llvm-commits

Differential Revision: https://reviews.llvm.org/D35439

llvm-svn: 313075