Revert "x/tools/container/intsets: remove unused function"

This reverts commit 79902b439b.

Reason for revert: build broken on some platforms.

Change-Id: I5ceddc305580ff97f47ae27da831f99e96dab424
Reviewed-on: https://go-review.googlesource.com/37539
Reviewed-by: David R. Jenni <david.r.jenni@gmail.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

container/intsets/util.go

@@ -4,6 +4,16 @@
 
 package intsets
 
+// From Hacker's Delight, fig 5.2.
+func popcountHD(x uint32) int {
+	x -= (x >> 1) & 0x55555555
+	x = (x & 0x33333333) + ((x >> 2) & 0x33333333)
+	x = (x + (x >> 4)) & 0x0f0f0f0f
+	x = x + (x >> 8)
+	x = x + (x >> 16)
+	return int(x & 0x0000003f)
+}
+
 var a [1 << 8]byte
 
 func init() {