all: calculate the median uniformly

This is a follow up of CL 526496.

src/internal/godebugs/table.go

@@ -55,7 +55,7 @@ func Lookup(name string) *Info {
 	lo := 0
 	hi := len(All)
 	for lo < hi {
-		m := lo + (hi-lo)>>1
+		m := int(uint(lo+hi) >> 1)
 		mid := All[m].Name
 		if name == mid {
 			return &All[m]

src/runtime/mranges.go

@@ -271,7 +271,7 @@ func (a *addrRanges) findSucc(addr uintptr) int {
 	const iterMax = 8
 	bot, top := 0, len(a.ranges)
 	for top-bot > iterMax {
-		i := ((top - bot) / 2) + bot
+		i := int(uint(bot+top) >> 1)
 		if a.ranges[i].contains(base.addr()) {
 			// a.ranges[i] contains base, so
 			// its successor is the next index.

src/syscall/syscall_openbsd.go

@@ -37,7 +37,7 @@ func nametomib(name string) (mib []_C_int, err error) {
 	left := 0
 	right := len(sysctlMib) - 1
 	for {
-		idx := left + (right-left)/2
+		idx := int(uint(left+right) >> 1)
 		switch {
 		case name == sysctlMib[idx].ctlname:
 			return sysctlMib[idx].ctloid, nil