Move name mangling before rewriting calls rather than after.
This is in preparation for later changes.

Change-Id: I74bc351f4290dad7ebf6d0d361bb684087786053
Reviewed-on: https://go-review.googlesource.com/c/142881
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>