vlog95: Add support for translating some power expressions as a left shift.
vlog95: Add support for translating some power expressions as a left shift.

If the left power operator expression is a numeric constant that has a
value 2^n n>0 then we can use a left shift to calculate the result. This
patch adds code to do this. All other power expressions are flagged as a
compilation error.

themperek/iverilog,themperek/iverilog,CastMi/iverilog,themperek/iverilog,CastMi/iverilog,CastMi/iverilog