[dev.ssa] cmd/compile/internal/ssa: add arg-dominating check, fix phielim

[dev.ssa] cmd/compile/internal/ssa: add arg-dominating check, fix phielim

Add a check to make sure value arguments dominate the value.

Phi elim output used to fail this test.  When eliminating
redundant phis, phi elim was using one of the args and not
the ultimate source.  For example:

          b1: x = ...
          -> b2 b3

b2: y = Copy x        b3: z = Copy x
-> b4                 -> b4

          b4: w = phi y z

Phi elim eliminates w, but it used to replace w with (Copy y).
That's bad as b2 does not dominate b4.  Instead we should
replace w with (Copy x).

Fixes #12347

Change-Id: I9f340cdabcda8e2e90359fb4f9250877b1fffe98
Reviewed-on: https://go-review.googlesource.com/13986
Reviewed-by: David Chase <drchase@google.com>