cmd/internal/gc: make use of new String methods in prints

cmd/internal/gc: make use of new String methods in prints

$ sam -d cmd/internal/gc/*.{go,y} cmd/?g/*.go
X ,s/, (gc\.)?[BFHNST]conv\(([^()]+), 0\)/, \2/g
X/'/w
q
$

Change-Id: Ic28a4807d237b8ae53ceca1e4e7fdb43580ab560
Reviewed-on: https://go-review.googlesource.com/9032
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Austin Clements <austin@google.com>