[MemorySSA] Make the use of moveAllAfterMergeBlocks consistent.
The rule for the moveAllAfterMergeBlocks API si for all instructions
from From to have been moved to To, while keeping the CFG edges (and
block terminators) unchanged.
Update all the callsites for moveAllAfterMergeBlocks to follow this.
Pending follow-up: since the same behavior is needed everytime, merge
all callsites into one. The common denominator may be the call to
Subscribers: Prazek, sanjoy.google, llvm-commits
Differential Revision: https://reviews.llvm.org/D68659