The pipeline parsing code was unnecessarily complex. It used a for loop
with a trailing break, a complex switch, and up to seven levels of
indentation.

Instead, drop the loop in favor of a single named goto with a comment,
and flatten out the complex switch to be easier to follow. Two lines of
code are now duplicated, but they're simple and only three lines apart.

While at it, move the pipe initialization further up to remove the need
for three variables.

Change-Id: I07b29de195f4000336219aadeadeacaaa4285c58
Reviewed-on: https://go-review.googlesource.com/c/145285Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>