cmd/compile: in poset, allow multiple aliases in a single pass

Change aliasnode into aliasnodes, to allow for recording
multiple aliases in a single pass. The nodes being aliased
are passed as bitset for performance reason (O(1) lookups).
It does look worse in the existing case of SetEqual where
we now need to allocate a bitset just for a single node,
but the new API will allow to fully implement a path-collapsing
primitive in next CL.

No functional changes, passes toolstash -cmp.

Change-Id: I06259610e8ef478106b36852464ed2caacd29ab5
Reviewed-on: https://go-review.googlesource.com/c/go/+/200860


Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Giovanni Bajo <rasky@develer.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>