cmd/compile: rewrite 2*x+c into LEAx1 on amd64

Rewrite x<<1+c into x+x+c, which can be expressed as a single LEAQ/LEAL.

Bit of a special case, but the single-instruction
LEA is both shorter and faster than SHL then ADD.

Triggers 293 times during make.bash.

Change-Id: I3f09c8e9a8f3859d1eeed336f095fc3ada79c2c1
Reviewed-on: https://go-review.googlesource.com/108938


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>