Add goto and labels and dialect conversions to MLIR

[robertoraggi]

Enough for

int goto_loop1() {
  int i = 0;

loop:
  if (i - 10) {
    i = i + 1;
    goto loop;
  }

  return i;
}

int goto_loop2() {
  int i;
  i = 10;
  goto L;
L:
  i = i + 1;
  i = i + 2;
  return i;
}

$ cxx x.c -emit-ir | mlir-translate -mlir-to-llvmir  |lli --entry-function=goto_loop1; echo $?
10

$ cxx x.c -emit-ir | mlir-translate -mlir-to-llvmir  |lli --entry-function=goto_loop2; echo $?
13