time: optimize big4

Use the same load order in big4 as in encoding/binary.BigEndian.
This order is recognized by the compiler and converted into single load.
This isn't in the hot path, but doesn't hurt readability, so lets do this.

Change-Id: Ib1240d0b278e9d667ad419fe91fa52b23d28cfc0
Reviewed-on: https://go-review.googlesource.com/130478
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/time/zoneinfo_read.go

@@ -55,7 +55,7 @@ func (d *dataIO) big4() (n uint32, ok bool) {
 		d.error = true
 		return 0, false
 	}
-	return uint32(p[0])<<24 | uint32(p[1])<<16 | uint32(p[2])<<8 | uint32(p[3]), true
+	return uint32(p[3]) | uint32(p[2])<<8 | uint32(p[1])<<16 | uint32(p[0])<<24, true
 }
 
 func (d *dataIO) byte() (n byte, ok bool) {