cmd/go: revert "remove unnecessary else conditions"

This reverts CL 144137.

Reason for revert: The justification for the original commit
was that golint said so, but golint is wrong. The code reads
more clearly the original way.

Change-Id: I960f286ed66fec67aabd953e7b69993f60b00bca
Reviewed-on: https://go-review.googlesource.com/c/149339


Reviewed-by: Russ Cox <rsc@golang.org>