Update #9855

In preparation for introducing direct use of a zero register on
platforms that support it, take the opportunity to clean up
Componentgen a bit.

Change-Id: I120ce1ffcca8c4f7603bfe76bfa1aedd27ebb4d2
Reviewed-on: https://go-review.googlesource.com/8691
Run-TryBot: Dave Cheney <dave@cheney.net>
Reviewed-by: Minux Ma <minux@golang.org>