math/big: save some copies in binaryGCD.

This patch resulted from a bit of quick optimisation in response to a
golang-nuts post. It looks like one could save a couple other copies in
this function, but this addresses the inner loop and is fairly simple.

benchmark                    old ns/op    new ns/op    delta
BenchmarkGCD10x10                 1964         1711  -12.88%
BenchmarkGCD10x100                2019         1736  -14.02%
BenchmarkGCD10x1000               2471         2171  -12.14%
BenchmarkGCD10x10000              6040         5778   -4.34%
BenchmarkGCD10x100000            43204        43025   -0.41%
BenchmarkGCD100x100              11004         8520  -22.57%
BenchmarkGCD100x1000             11820         9446  -20.08%
BenchmarkGCD100x10000            23846        21382  -10.33%
BenchmarkGCD100x100000          133691       131505   -1.64%
BenchmarkGCD1000x1000           120041        95591  -20.37%
BenchmarkGCD1000x10000          136887       113600  -17.01%
BenchmarkGCD1000x100000         295370       273912   -7.26%
BenchmarkGCD10000x10000        2556126      2205198  -13.73%
BenchmarkGCD10000x100000       3159512      2808038  -11.12%
BenchmarkGCD100000x100000    150543094    139986045   -7.01%

R=gri, remyoudompheng
CC=bradfitz, gobot, golang-dev, gri
https://golang.org/cl/9424043

src/pkg/math/big/int.go

@@ -703,14 +703,15 @@ func (z *Int) binaryGCD(a, b *Int) *Int {
 		// reduce t
 		t.Rsh(t, t.abs.trailingZeroBits())
 		if t.neg {
-			v.Neg(t)
+			v, t = t, v
+			v.neg = len(v.abs) > 0 && !v.neg // 0 has no sign
 		} else {
-			u.Set(t)
+			u, t = t, u
 		}
 		t.Sub(u, v)
 	}
 
-	return u.Lsh(u, k)
+	return z.Lsh(u, k)
 }
 
 // ProbablyPrime performs n Miller-Rabin tests to check whether x is prime.