One could expect calls like

  z.mant.shl(z.mant, shiftAmount)

(or higher-level-functions calls that use lhs/rhs) to be almost free
when shiftAmount = 0; and expect calls like

  z.mant.shl(x.mant, 0)

to have the same cost of a x.mant -> z.mant copy. Neither of this
things are currently true.

For an 800 words nat, the first kind of calls cost ~800ns for rigth
shifts and ~3.5µs for left shift; while the second kind of calls are
doing more work than necessary by calling shlVU/shrVU.

This change makes the first kind of calls ({Shl,Shr}Same) almost free,
and the second kind of calls ({Shl,Shr}) about 30% faster.

name                  old time/op  new time/op  delta
ZeroShifts/Shl-4      3.64µs ± 3%  2.49µs ± 1%  -31.55%  (p=0.000 n=10+10)
ZeroShifts/ShlSame-4  3.65µs ± 1%  0.01µs ± 1%  -99.85%  (p=0.000 n=9+9)
ZeroShifts/Shr-4      3.65µs ± 1%  2.49µs ± 1%  -31.91%  (p=0.000 n=10+10)
ZeroShifts/ShrSame-4   825ns ± 0%     6ns ± 1%  -99.33%  (p=0.000 n=9+10)

During go test math/big, the shl zeroshift fastpath is triggered 1380
times; while the shr fastpath is triggered 153334 times(!).

Change-Id: I5f92b304a40638bd8453a86c87c58e54b337bcdf
Reviewed-on: https://go-review.googlesource.com/87660
Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>