math/big: write t*10 to multiply t by 10

math/big: write t*10 to multiply t by 10

The compiler has caught up.
In fact the compiler is ahead; it knows about a magic multiply-by-5 instruction:

	// compute '0' + byte(r - t*10) in AX
	MOVQ	t, AX
	LEAQ	(AX)(AX*4), AX
	SHLQ	$1, AX
	MOVQ	r, CX
	SUBQ	AX, CX
	LEAL	48(CX), AX

For comparison, the shifty version compiles to:

	// compute '0' + byte(r - t*10) in AX
	MOVQ	t, AX
	MOVQ	AX, CX
	SHLQ	$3, AX
	MOVQ	r, DX
	SUBQ	AX, DX
	SUBQ	CX, DX
	SUBQ	CX, DX
	LEAL	48(DX), AX

Fixes #2671.

Change-Id: Ifbf23dbfeb19c0bb020fa44eb2f025943969fb6b
Reviewed-on: https://go-review.googlesource.com/23372
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>