Fix profile collection/merge

Bug: http://b/37574207

Previously pgo_profdata_file returned None if the file didn't exist.
This will fail when this function is queried during profile file
creation itself.  Pull file-existence check out to just the necessary
callers.

Test: test_compiler.py with --generate-clang-profile
Change-Id: I402f14a1ef985d5bc953be3a2e44a8275862d954