[GVN,NewGVN] Move patchReplacementInstruction to Utils/Local.h

This function is shared between both implementations. I am not sure if
Utils/Local.h is the best place though.

Reviewers: davide, dberlin, efriedma, xbolva00

Reviewed By: efriedma, xbolva00

Differential Revision: https://reviews.llvm.org/D47337

llvm-svn: 339138