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.

Change-Id: I4fb402fc684ef4c543fc65aea343ca1a4d73a189
Reviewed-on: https://go-review.googlesource.com/c/151979


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>