[PM] Teach the PGO instrumentation pasess to run GlobalDCE before

instrumenting code.

This is important in the new pass manager. The old pass manager's
inliner has a small DCE routine embedded within it. The new pass manager
relies on the actual GlobalDCE pass for this.

Without this patch, instrumentation profiling with the new PM results in
massive code bloat in the object files because the instrumentation
itself ends up preventing DCE from working to remove the code.

We should probably change the instrumentation (and/or DCE) so that we
can eliminate dead code even if instrumented, but we shouldn't even
spend the time generating instrumentation for that code so this still
seems like a good patch.

Differential Revision: https://reviews.llvm.org/D33535

llvm-svn: 303845