cmd/compile: insertVarPhis micro-optimization

cmd/compile: insertVarPhis micro-optimization

Algorithmic improvements here are hard.
Lifting a lookup out of the loop helps a little, though.

To compile the code in #17926:

name  old s/op   new s/op   delta
Real   146 ± 3%   140 ± 4%  -3.87%  (p=0.002 n=10+10)
User   143 ± 3%   139 ± 4%  -3.08%  (p=0.005 n=10+10)
Sys   8.28 ±35%  8.08 ±28%    ~     (p=0.684 n=10+10)

Updates #17926.

Change-Id: Ic255ac8b7b409c1a53791058818b7e2cf574abe3
Reviewed-on: https://go-review.googlesource.com/33305
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>