[dev.regabi] cmd/compile: replace evconst with non-mutating version

evconst is one of the largest sources of Op rewrites,
which prevent separating different kinds of nodes
(in this case, arithmetic nodes and OLITERAL nodes).

The change in swt.go is necessary because otherwise
the syntax graph ends up containing that OLEN expression
multiple times, which violates the invariant that it's a tree
except for ONAME, OLITERAL, and OTYPE nodes.
(Before, the OLEN was overwritten by an OLITERAL, so the
invariant still held, but now that we don't overwrite it,
we need a different copy for each instance.)

Passes toolstash -cmp.

Change-Id: Ia004774ab6852fb384805d0f9f9f234b40842811
Reviewed-on: https://go-review.googlesource.com/c/go/+/272869


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>