Since switching to SSA, the only remaining use for the Ullman field
was in tracking whether or not an expression contained a function
call. Give it a new name and encode it in our fancy new bitset field.

Passes toolstash-check.

Change-Id: I95b7f9cb053856320c0d66efe14996667e6011c2
Reviewed-on: https://go-review.googlesource.com/37721
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>