Fix modcc simd generation (#1681)

Fixes 2 bugs where modcc expects a range variable but gets a non-range variable instead.

    PowBinaryExpressions were being handled separately from other BinaryExpressions and as a result, some crucial analysis was being skipped on the analysis of the lhs and rhs arguments. This PR incorporates the analysis and printing of PowBinaryExpression into BinaryExpression.
    Printing code for a masked AssignmentExpression was not handling the case of a non-range variable on the rhs of the assignment correctly. In that case, an explicit cast to a vector is needed, which is added in this PR.