Fix Calyx backend issue with emitting signed binops

[Mark1626]

This fixes point number 2 in #427

Background

Dahlia was emitting Calyx primitives incorrectly for signed ops.

eg)

decl a: bit<10>[128];
decl b: bit<10>[128];
decl c: bit<10>[128];

for (let i:bit<8>=0..128) {
    c[i] := a[i] & b[i];
}

The Calyx IR contains a std_sand primitive

Changes Summary

1. Modify the function signed in Helpers.scala to use the op to decide if a given op is signed or not
   1. Match for op type NumOp and use the previous behaviour
   2. Any other op type binary operator primitives are not signed eg) bitwise, comparison
2. Add an test with signed bit and binops for regression

[rachitnigam]

LGTM!