math/bits: optimize Reverse32 and Reverse64

Use ReverseBytes32 and ReverseBytes64 to speed up these functions.
The byte reversal functions are intrinsics on most platforms and
generally compile to a single instruction.

name       old time/op  new time/op  delta
Reverse32  2.41ns ± 1%  1.94ns ± 3%  -19.60%  (p=0.000 n=20+19)
Reverse64  3.85ns ± 1%  2.56ns ± 1%  -33.32%  (p=0.000 n=17+19)

Change-Id: I160bf59a0c7bd5db94114803ec5a59fae448f096
Reviewed-on: https://go-review.googlesource.com/c/159358
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/math/bits/bits.go

@@ -232,8 +232,7 @@ func Reverse32(x uint32) uint32 {
 	x = x>>1&(m0&m) | x&(m0&m)<<1
 	x = x>>2&(m1&m) | x&(m1&m)<<2
 	x = x>>4&(m2&m) | x&(m2&m)<<4
-	x = x>>8&(m3&m) | x&(m3&m)<<8
-	return x>>16 | x<<16
+	return ReverseBytes32(x)
 }
 
 // Reverse64 returns the value of x with its bits in reversed order.
@@ -242,9 +241,7 @@ func Reverse64(x uint64) uint64 {
 	x = x>>1&(m0&m) | x&(m0&m)<<1
 	x = x>>2&(m1&m) | x&(m1&m)<<2
 	x = x>>4&(m2&m) | x&(m2&m)<<4
-	x = x>>8&(m3&m) | x&(m3&m)<<8
-	x = x>>16&(m4&m) | x&(m4&m)<<16
-	return x>>32 | x<<32
+	return ReverseBytes64(x)
 }
 
 // --- ReverseBytes ---