Fix bit shifts with bit literals on lhs

[orpuente-MS]

The following program was failing to compile because the 1 on the lhs of the << expression was being lowered as an int, and the qasm spec does not define bit shifts for ints. The fix consists in lowering the literal as a uint instead, which is allowed by the spec.

const int a = 1 << 5;
def const_evaluation_context() {
    int b = a;
}