[dev.regabi] cmd/compile: remove uses of Name.Offset, Name.copy

For globals, Name.Offset is used as a way to address a field within
a global during static initialization. This CL replaces that use with
a separate NameOffsetExpr (ONAMEOFFSET) node.

For locals, Name.Offset is the stack frame offset. This CL calls it
that (FrameOffset, SetFrameOffset).

Now there is no longer any use of Name.Offset or Name.SetOffset.

And now that copies of Names are not being made to change their
offsets, we can lock down use of ir.Copy on Names. The only
remaining uses are during inlining and in handling generic system
functions. At both those times you do want to create a new name
and that can be made explicit by calling the new CloneName method
instead. ir.Copy on a name now panics.

Passes buildall w/ toolstash -cmp.

Change-Id: I0b0a25b9d93aeff7cf4e4025ac53faec7dc8603b
Reviewed-on: https://go-review.googlesource.com/c/go/+/278914


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>