This further reduces the number of flags spills
during make.bash by about 50%.

Note that GetG is implemented by one or two MOVs,
which is why it does not clobber flags.

Change-Id: I6fede8c027b7dc340e00d1e15df1b87bf2b2d9ec
Reviewed-on: https://go-review.googlesource.com/13843Reviewed-by: Keith Randall <khr@golang.org>