cmd/internal/obj: add Prog.SetFrom3{Reg,Const}

These are the the most common uses, and they reduce line noise.

I don't love adding new deprecated APIs,
but since they're trivial wrappers,
it'll be very easy to update them along with the rest.
No functional changes; passes toolstash-check.

Change-Id: I691a8175cfef9081180e463c63f326376af3f3a6
Reviewed-on: https://go-review.googlesource.com/c/go/+/296009


Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>