[MemCpyOpt] Skip optimizing basic blocks not reachable from entry

Summary:
Skip basic blocks not reachable from the entry node
in MemCpyOptPass::iterateOnFunction.

Code that is unreachable may have properties that do not exist
for reachable code (an instruction in a basic block can for
example be dominated by a later instruction in the same basic
block, for example if there is a single block loop).
MemCpyOptPass::processStore is only safe to use for reachable
basic blocks, since it may iterate past the basic block
beginning when used for unreachable blocks. By simply skipping
to optimize unreachable basic blocks we can avoid asserts such
as "Assertion `!NodePtr->isKnownSentinel()' failed."
in MemCpyOptPass::processStore.

The problem was detected by fuzz tests.

Reviewers: eli.friedman, dneilson, efriedma

Reviewed By: efriedma

Subscribers: efriedma, llvm-commits

Differential Revision: https://reviews.llvm.org/D45889

llvm-svn: 330635