PGO: Add a run-make test that makes sure that PGO profiling data is used by the compiler during optimizations.
From the tests comment section:
```
# This test makes sure that PGO profiling data leads to cold functions being
# marked as `cold` and hot functions with `inlinehint`.
# The test program contains an `if` were actual execution only ever takes the
# `else` branch. Accordingly, we expect the function that is never called to
# be marked as cold.
```
r? @alexcrichton
bors
7c71bc3208