[llvm][IPO] Inclusive language: Rename mergefunc-sanity to mergefunc-verify...

[llvm][IPO] Inclusive language: Rename mergefunc-sanity to mergefunc-verify and remove other instances of sanity in MergeFunctions.cpp

This patch renames the mergefunc-sanity to mergefunc-verify and renames the related functions to use more
inclusive language

Reviewed By: cebowleratibm

Differential Revision: https://reviews.llvm.org/D114374