[llvm-reduce] Add reduction for aliases.

This patch adds a new reduction pass that tries to remove aliases.
It runs early, as most of those likely can be removed up-front in
practice.

This substantially improves llvm-reduce for IR generated by the swift
compiler, which can generate a lot of aliases which lead to lots of
invalid reductions.

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D90260