[dev.regabi] cmd/compile: remove some unneeded code in package ir

The deepCopy functions haven't been needed since we switched to using
Edit everywhere, and AddStringExpr no longer has an Alloc field that
needs special casing.

Passes toolstash -cmp.

Change-Id: I5bcc8c73d5cb784f7e57fb3162ae6e288e6c9392
Reviewed-on: https://go-review.googlesource.com/c/go/+/280445


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>