PGO: rename FileCheck variable to follow the existing convention.

I added this "STF" variable without noticing that all the other counter names
end with a "C". Renaming it to "STC" for consistency.
PGO: rename profile data files from .pgodata to .profdata.

These tests were added before we had settled on using a .profdata extension
for the profile data files. Renaming them now for consistency.