MSan instrumentation is driven by the original code. We take every
incoming instruction and emit another instruction (or ten) next to
it, operating on the shadow values (but sometimes on the real values,
too). Two programs in one, essentially. There can be any kinds of
redundancies in the second one, so we just run whatever is normally
run at -O2, and then exclude some passes that do not help much with
benchmarks.

llvm-svn: 174049