cmd/compile: generate commutative rules when a condition is present

The commutative rule generator has an optimization
where given (Add x y) with no other uses of x and y in the matching rule,
it doesn't generate the commutative match (Add y x).

However, if there is also a condition referring to x or y,
such as (Add x y) && isFoo(x), then we should generate the commutative rule.
This change parses the condition, extracts all idents, and takes them
into consideration.

This doesn't yield any new optimizations now.
However, it is the right thing to do;
otherwise we'll have to track it down and fix it again later.

It is also expensive now, in terms of additional generated code.
However, it will be much, much less expensive soon,
once our generated code for commutative ops gets smaller.

Change-Id: I52c2016c884bbc7789bf8dfe9b9c56061bc028ad
Reviewed-on: https://go-review.googlesource.com/c/go/+/213702


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