cmd/compile: fix reslice

:= is the wrong thing here.  The new variable masks the old
variable so we allocate the slice afresh each time around the loop.

Change-Id: I759c30e1bfa88f40decca6dd7d1e051e14ca0844
Reviewed-on: https://go-review.googlesource.com/22679
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>

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

@@ -2065,7 +2065,7 @@ func (s *regAllocState) computeLive() {
 
 			// Propagate backwards to the start of the block
 			// Assumes Values have been scheduled.
-			phis := phis[:0]
+			phis = phis[:0]
 			for i := len(b.Values) - 1; i >= 0; i-- {
 				v := b.Values[i]
 				live.remove(v.ID)