SSA: function parameter use index 1 instead of 0

```solidity
contract A{
function f(int a, int b) public returns(int){
return a+b;
}
}
```
$ `slither test.sol --print slithir-ssa`

Function A.f(int256,int256)
Expression: a + b
IRs:
TMP_0(int256) = a_1 (c)+ b_1
RETURN TMP_0
```

Here `a_1` should be `a_0`. In particular this will break the taint analysis on the SSA API