Add CFG simplification

concatenation of block which has at most one instruction to other blocks that jump to it
test code:
let rec fibt x a b = if x <= 0 then a else fibt (x - 1) (a + b) a in let rec fib n = fibt n 1 0 in fib 5
Fix SSASimpl.hs

Add restriction to the set of merged blocks (blocks with TRet only)