cmd/compile: call phiElimValue from removePhiArg

cmd/compile: call phiElimValue from removePhiArg

With the exception of the shortcircuit pass, removePhiArg is always unconditionally followed by phiElimValue.
Move the phiElimValue inside removePhiArg.

Resolves a TODO.

See CL 357964 for more info.

Change-Id: I8460b35864f4cd7301ba86fc3dce08ec8041da7f
Reviewed-on: https://go-review.googlesource.com/c/go/+/465435
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Jakub Ciolek <jakub@ciolek.dev>