cmd/compile: add Buffer.Grow to TestIntendedInlining
golang.org/cl/151977 slightly decreased the cost of inlining an extra
call from 60 to 57, since it was a safe change that could help in some
scenarios.
One notable change spotted in that CL is that bytes.Buffer.Grow is now
inlinable, meaning that a fixedbugs test needed updating.
For consistency, add the test case to TestIntendedInlining too,
alongside other commonly used bytes.Buffer methods.