Fix for GitHub issue 14 : Bug in processing 1'b1 >= |1'bx.

During expression evaluation, the compiler attempts to optimise away
relational operations when one side is constant and all possible values
of the other side would result in the relation being true. This is not
a valid optimisation if the other side is a 4-state variable, as an
'x' or 'z' will result in the relation being unknown.