cmd/compile: Use pre-regalloc value ID in lateSpillUse

The cached copy's ID is sometimes outside the bounds of the orig array.

There's no reason to start at the cached copy and work backwards
to the original value. We already have the original value ID at
all the callsites.

Fixes noopt build

Change-Id: I313508a1917e838a87e8cc83b2ef3c2e4a8db304
Reviewed-on: https://go-review.googlesource.com/22355
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -1540,9 +1540,9 @@ func (s *regAllocState) isLoopSpillCandidate(loop *loop, v *Value) bool {
 	return s.values[v.ID].needReg && !s.values[v.ID].spillUsed && s.loopnest.b2l[v.Block.ID] == loop
 }
 
-// lateSpillUse notes a late (after stack allocation) use of spill c
+// lateSpillUse notes a late (after stack allocation) use of the spill of value with ID vid.
 // This will inhibit spill sinking.
-func (s *regAllocState) lateSpillUse(c *Value) {
+func (s *regAllocState) lateSpillUse(vid ID) {
 	// TODO investigate why this is necessary.
 	// It appears that an outside-the-loop use of
 	// an otherwise sinkable spill makes the spill
@@ -1551,10 +1551,7 @@ func (s *regAllocState) lateSpillUse(c *Value) {
 	// true when isLoopSpillCandidate was called, yet
 	// it was shuffled).  Such shuffling cuts the amount
 	// of spill sinking by more than half (in make.bash)
-	v := s.orig[c.ID]
-	if v != nil {
-		s.values[v.ID].spillUsedShuffle = true
-	}
+	s.values[vid].spillUsedShuffle = true
 }
 
 // shuffle fixes up all the merge edges (those going into blocks of indegree > 1).
@@ -1729,7 +1726,7 @@ func (e *edgeState) process() {
 		if _, isReg := loc.(*Register); isReg {
 			c = e.p.NewValue1(c.Line, OpCopy, c.Type, c)
 		} else {
-			e.s.lateSpillUse(c)
+			e.s.lateSpillUse(vid)
 			c = e.p.NewValue1(c.Line, OpLoadReg, c.Type, c)
 		}
 		e.set(r, vid, c, false)
@@ -1818,7 +1815,7 @@ func (e *edgeState) processDest(loc Location, vid ID, splice **Value) bool {
 			}
 		} else {
 			if dstReg {
-				e.s.lateSpillUse(c)
+				e.s.lateSpillUse(vid)
 				x = e.p.NewValue1(c.Line, OpLoadReg, c.Type, c)
 			} else {
 				// mem->mem. Use temp register.
@@ -1836,7 +1833,7 @@ func (e *edgeState) processDest(loc Location, vid ID, splice **Value) bool {
 				e.erase(loc)
 
 				r := e.findRegFor(c.Type)
-				e.s.lateSpillUse(c)
+				e.s.lateSpillUse(vid)
 				t := e.p.NewValue1(c.Line, OpLoadReg, c.Type, c)
 				e.set(r, vid, t, false)
 				x = e.p.NewValue1(c.Line, OpStoreReg, loc.(LocalSlot).Type, t)