Add unstructured conditional branches

[robertoraggi]

In this change we convert if, while and do statements using unstructured control flow. Obviously, the plan is to produce structured control flow but we will do that later.

int while_loop() {
  int x = 10;
  while (x) x = x - 1;
  return x;
}

int do_loop() {
  int x = 10;
  do x = x - 1;
  while (x);
  return x;
}

int cond1() {
  int x = 0;
  if (!x)
    x = 10;
  else
    x = 20;
  return x;
}

int cond2() {
  int x = 1;
  if (!x)
    x = 10;
  else
    x = 20;
  return x;
}

$ cxx x.c -emit-ir | mlir-translate -mlir-to-llvmir  |lli --entry-function=while_loop; echo $?
0

$ cxx x.c -emit-ir | mlir-translate -mlir-to-llvmir  |lli --entry-function=do_loop; echo $?
0

$ cxx x.c -emit-ir | mlir-translate -mlir-to-llvmir  |lli --entry-function=cond1; echo $?  
10