[MergedLoadStoreMotion] Fix a debug invariant bug in mergeStores

[MergedLoadStoreMotion] Fix a debug invariant bug in mergeStores

Summary:
MergedLoadStoreMotion::mergeStores is using some heuristics
to limit the amount of stores that it tries to sink (see
MagicCompileTimeControl in MergedLoadStoreMotion.cpp). The
heuristic involves counting the number of instructions in
one of the basic blocks that is part of the transformation.

We now ignore dbg intrinsics when counting instruction for
the MagicCompileTimeControl heuristic. This to make sure that
the amount of stores that are sunk doesn't depend on the amount
of debug information (if -g is used or not).

Reviewers: Gerolf, davide, majnemer

Reviewed By: davide

Subscribers: dberlin, bjope, aprantl, JDevlieghere, llvm-commits

Differential Revision: https://reviews.llvm.org/D46600

llvm-svn: 331852