math/big: slightly faster float->decimal conversion

Inspired by Alberto Donizetti's observations in
https://go-review.googlesource.com/#/c/30099/.

name                 old time/op  new time/op  delta
DecimalConversion-8   138µs ± 1%   136µs ± 2%  -1.85%  (p=0.000 n=10+10)

10 runs each, measured on a Mac Mini, 2.3 GHz Intel Core i7.

Performance improvements varied between -1.25% to -4.4%; -1.85% is
about in the middle of the observed improvement. The generated code
is slightly shorter in the inner loops of the conversion code.

Change-Id: I10fb3b2843da527691c39ad5e5e5bd37ed63e2fa
Reviewed-on: https://go-review.googlesource.com/31250Reviewed-by: Alan Donovan <adonovan@google.com>

src/math/big/decimal.go

@@ -125,11 +125,12 @@ func shr(x *decimal, s uint) {
 
 	// read a digit, write a digit
 	w := 0 // write index
+	mask := Word(1)<<s - 1
 	for r < len(x.mant) {
 		ch := Word(x.mant[r])
 		r++
 		d := n >> s
-		n -= d << s
+		n &= mask // n -= d << s
 		x.mant[w] = byte(d + '0')
 		w++
 		n = n*10 + ch - '0'
@@ -138,7 +139,7 @@ func shr(x *decimal, s uint) {
 	// write extra digits that still fit
 	for n > 0 && w < len(x.mant) {
 		d := n >> s
-		n -= d << s
+		n &= mask
 		x.mant[w] = byte(d + '0')
 		w++
 		n = n * 10
@@ -148,7 +149,7 @@ func shr(x *decimal, s uint) {
 	// append additional digits that didn't fit
 	for n > 0 {
 		d := n >> s
-		n -= d << s
+		n &= mask
 		x.mant = append(x.mant, byte(d+'0'))
 		n = n * 10
 	}