Move memory coercion functions from GVN.cpp to VNCoercion.cpp so they can be...

Move memory coercion functions from GVN.cpp to VNCoercion.cpp so they can be shared between GVN and NewGVN.

Summary:
These are the functions used to determine when values of loads can be
extracted from stores, etc, and to perform the necessary insertions to
do this.  There are no changes to the functions themselves except
reformatting, and one case where memdep was informed of a removed load
(which was pushed into the caller).

Reviewers: davide

Subscribers: mgorny, llvm-commits, Prazek

Differential Revision: https://reviews.llvm.org/D30478

llvm-svn: 297438