At this point in the compiler we haven't assigned Xoffset values for
PAUTO variables anyway, so just immediately store the stack offsets
into Xoffset rather than into a global map.

Change-Id: I61eb471c857c8b145fd0895cbd98fd4e8d3c3365
Reviewed-on: https://go-review.googlesource.com/30081
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>