Improve loop bound detection for loop unrolling by handling casts in loop statements

[karoliineh]

Inspired by an unreach-call task num_conversion_1.c where we couldn't detect a bound for the following loop:

unsigned char c;
c = 0;
while (c < (unsigned char) 8) {
  ...
}

Unrolling the loop 8 times here helps us solve the task. In addition to the case in this program, where the loop statement is in the form CastE(TInt(int, ), Lval(Var(c, NoOffset))) < Const(Int(8,int,None)) I included all possible combinations of casting for both the left and right expressions in the loop comparison statement.