cmd/compile: reenable phielim during rewrite

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>

src/cmd/compile/internal/ssa/phielim.go

@@ -40,11 +40,7 @@ func phielimValue(v *Value) bool {
 	// are not v itself, then the phi must remain.
 	// Otherwise, we can replace it with a copy.
 	var w *Value
-	for i, x := range v.Args {
-		if b := v.Block.Preds[i]; b.Kind == BlockFirst && b.Succs[1] == v.Block {
-			// This branch is never taken so we can just eliminate it.
-			continue
-		}
+	for _, x := range v.Args {
 		if x == v {
 			continue
 		}

src/cmd/compile/internal/ssa/rewrite.go

@@ -40,6 +40,8 @@ func applyRewrite(f *Func, rb func(*Block) bool, rv func(*Value, *Config) bool)
 			}
 			curb = nil
 			for _, v := range b.Values {
+				change = phielimValue(v) || change
+
 				// Eliminate copy inputs.
 				// If any copy input becomes unused, mark it
 				// as invalid and discard its argument. Repeat