cmd/compile: use LZCNT instruction for GOAMD64>=3

cmd/compile: use LZCNT instruction for GOAMD64>=3

LZCNT is similar to BSR, but BSR(x) is undefined when x == 0, so using
LZCNT can avoid a special case for zero input. Except that case,
LZCNTQ(x) == 63-BSRQ(x) and LZCNTL(x) == 31-BSRL(x).

And according to https://www.agner.org/optimize/instruction_tables.pdf,
LZCNT instructions are much faster than BSR on AMD CPU.

name              old time/op  new time/op  delta
LeadingZeros-8    0.91ns ± 1%  0.80ns ± 7%  -11.68%  (p=0.000 n=9+9)
LeadingZeros8-8   0.98ns ±15%  0.91ns ± 1%   -7.34%  (p=0.000 n=9+9)
LeadingZeros16-8  0.94ns ± 3%  0.92ns ± 2%   -2.36%  (p=0.001 n=10+10)
LeadingZeros32-8  0.89ns ± 1%  0.78ns ± 2%  -12.49%  (p=0.000 n=10+10)
LeadingZeros64-8  0.92ns ± 1%  0.78ns ± 1%  -14.48%  (p=0.000 n=10+10)

Change-Id: I125147fe3d6994a4cfe558432780408e9a27557a
Reviewed-on: https://go-review.googlesource.com/c/go/+/396794
Reviewed-by: Keith Randall <khr@golang.org>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
TryBot-Result: Gopher Robot <gobot@golang.org>