cmd/compile: factor out code to remove phi argument

CL 358117 fixed a bug that Phi's argument wasn't updated correctly after
removing a predecessor of Block. This CL factor out the code that
updates phi argument into a Block's method, so it's easier to use,
maintain and hopefully prevent that kind of bug in the future.

Change-Id: Ie9741e19ea28f56860425089b6093a381aa10f5b
Reviewed-on: https://go-review.googlesource.com/c/go/+/357964


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>