[dev.ssa] cmd/compile: strength-reduce 64-bit constant divides

[dev.ssa] cmd/compile: strength-reduce 64-bit constant divides

The frontend does this for 32 bits and below, but SSA needs
to do it for 64 bits.  The algorithms are all copied from
cgen.go:cgen_div.

Speeds up TimeFormat substantially: ~40% slower to ~10% slower.

Change-Id: I023ea2eb6040df98ccd9105e15ca6ea695610a7a
Reviewed-on: https://go-review.googlesource.com/19302
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Todd Neal <todd@tneal.org>