[mem2reg] Use range loops (NFCI)

Summary:
Several for loops in PromoteMemoryToRegister.cpp leave their increment
expression empty, instead incrementing the iterator within the for loop
body. I believe this is because these loops were previously implemented
as while loops; see https://reviews.llvm.org/rL188327.

Incrementing the iterator within the body of the for loop instead of
in its increment expression makes it seem like the iterator will be
modified or conditionally incremented within the loop, but that is not
the case in these loops.

Instead, use range loops.

Test Plan: `check-llvm`

Reviewers: davide, bkramer

Reviewed By: davide, bkramer

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D43473

llvm-svn: 325532