Implement addressof and dereference operators in codegen

[robertoraggi]

int ptr_1() {
  int i = 42;
  int *p = &i;
  return *p;
}

int ptr_2() {
  int i = 42;
  int *p = &i;
  i = 123;
  return *p;
}

int ptr_3() {
  int i = 123;
  return *&i;
}

$ x.c -emit-ir -toolchain macos |mlir-translate --mlir-to-llvmir |lli --entry-function=ptr_1; echo $?
42

$ x.c -emit-ir -toolchain macos |mlir-translate --mlir-to-llvmir |lli --entry-function=ptr_2; echo $?
123

$ x.c -emit-ir -toolchain macos |mlir-translate --mlir-to-llvmir |lli --entry-function=ptr_3; echo $?
123