Optimise Away Constraints Which Always Pass

[DavePearce]

In mir/lower.go vanishing constraints are added after constant propagation is performed on their expressions. However, in some cases, these might evaluate to 0 and hence can be dropped altogether. For example:

(column X)
(vanish tmp (if 0 0 X))

Generates the following:

(column #0.X)
(vanish tmp 0)
(vanish tmp 0)

Clearly, we don't need to do anything here.