runtime: special case allocation of arrays of size 1

This avoids division and multiplication.
Instrumentation suggests that this is a very common case.

Change-Id: I2d5d5012d4f4df4c4af1f9f85ca9c323c9889c0e
Reviewed-on: https://go-review.googlesource.com/54657
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/malloc.go

@@ -847,6 +847,9 @@ func reflect_unsafe_New(typ *_type) unsafe.Pointer {
 
 // newarray allocates an array of n elements of type typ.
 func newarray(typ *_type, n int) unsafe.Pointer {
+	if n == 1 {
+		return mallocgc(typ.size, typ, true)
+	}
 	if n < 0 || uintptr(n) > maxSliceCap(typ.size) {
 		panic(plainError("runtime: allocation size out of range"))
 	}