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>

src/math/big/natconv.go

@@ -391,7 +391,7 @@ func (q nat) convertWords(s []byte, b Word, ndigits int, bb Word, table []diviso
 				// this appears to be faster for BenchmarkString10000Base10
 				// and smaller strings (but a bit slower for larger ones)
 				t := r / 10
-				s[i] = '0' + byte(r-t<<3-t-t) // TODO(gri) replace w/ t*10 once compiler produces better code
+				s[i] = '0' + byte(r-t*10)
 				r = t
 			}
 		}