Remove the "optimization" that was causing the issue.

For the following code the "optimization" was
converting v to (OpCopy x) which is wrong because
x doesn't dominate v.

b1:
    y = ...
    First .. b3
b2:
   x = ...
   Goto b3
b3:
   v = phi x y
   ... use v ...

That "optimization" is likely no longer needed because
we now have a second opt pass with a dce in between
which removes blocks of type First.

For pkg/tools/linux_amd64/* the binary size drops
from 82142886 to 82060034.

Change-Id: I10428abbd8b32c5ca66fec3da2e6f3686dddbe31
Reviewed-on: https://go-review.googlesource.com/22312
Run-TryBot: Alexandru Moșoi <alexandru@mosoi.ro>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>