Add unstructured control flow for for-loop, break and continue

[robertoraggi]

int for_loop() {
  int x = 0;
  for (int i = 10; i; i = i - 1) x = x + 1;
  return x;
}

int break_loop() {
  int x = 0;
  for (int i = 10; i; i = i - 1) {
    if (!(i - 5)) break;
    x = x + 1;
  }
  return x;
}

int continue_loop() {
  int x = 0;
  for (int i = 10; i; i = i - 1) {
    if (!(i - 5)) continue;
    x = x + 1;
  }
  return x;
}

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

$ cxx x.c -emit-ir | mlir-translate -mlir-to-llvmir  |lli --entry-function=break_loop; echo $?
5

$ cxx x.c -emit-ir | mlir-translate -mlir-to-llvmir  |lli --entry-function=continue_loop; echo $?
9