cmd/compile: use math/bits for bvec

And delete some dead code.

Minor cleanup. Passes toolstash-check.

Change-Id: Ia3c77c6bf14942654d00d125a6221e63a442f3c6
Reviewed-on: https://go-review.googlesource.com/c/go/+/227317
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/bv.go

@@ -4,6 +4,10 @@
 
 package gc
 
+import (
+	"math/bits"
+)
+
 const (
 	wordBits  = 32
 	wordMask  = wordBits - 1
@@ -108,30 +112,11 @@ func (bv bvec) Next(i int32) int32 {
 
 	// Find 1 bit.
 	w := bv.b[i>>wordShift] >> uint(i&wordMask)
-
+	i += int32(bits.TrailingZeros32(w))
-	for w&1 == 0 {
-		w >>= 1
-		i++
-	}
 
 	return i
 }
 
-// Len returns the minimum number of bits required to represent bv.
-// The result is 0 if no bits are set in bv.
-func (bv bvec) Len() int32 {
-	for wi := len(bv.b) - 1; wi >= 0; wi-- {
-		if w := bv.b[wi]; w != 0 {
-			for i := wordBits - 1; i >= 0; i-- {
-				if w>>uint(i) != 0 {
-					return int32(wi)*wordBits + int32(i) + 1
-				}
-			}
-		}
-	}
-	return 0
-}
-
 func (bv bvec) IsEmpty() bool {
 	for _, x := range bv.b {
 		if x != 0 {