Use only last sub-expression of Expressions nodes for conditional type filters

[HertzDevil]

Fixes the following run-time error:

if (x = nil; true)
  p x # => can't execute `p(x)` at eval:2:3: `x` has no type (Exception)
end

This happens because Crystal computes the type filter from all sub-expressions of the (x = nil; true) node, and x = nil produces a truthy filter on x, even though the Nil type is never truthy; thus flow typing gives x no type inside the if-clause. This PR restricts the filter computation to the last sub-expression.

Similar errors can be triggered with while expressions; #10350 probably requires this PR.