Add, Sub, Mul,and bool conversions with lowering patterns

[robertoraggi]

barely enough for:

int add() { return 1 + 2; }
int sub() { return 5 - 3; }
int mul() { return 4 * 2; }
int unary_not() { return !true; }
int unary_not1() { return !false; }
bool to_bool() {
  int i = 0;
  return i;
}
int to_int() {
  bool b = true;
  return b;
}

$ cxx x.c -emit-ir | mlir-translate -mlir-to-llvmir  |lli --entry-function=add; echo $? 
3

$ cxx x.c -emit-ir | mlir-translate -mlir-to-llvmir  |lli --entry-function=sub; echo $?
2

$ cxx x.c -emit-ir | mlir-translate -mlir-to-llvmir  |lli --entry-function=mul; echo $?
8

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

$ cxx x.c -emit-ir | mlir-translate -mlir-to-llvmir  |lli --entry-function=unary_not1; echo $?
1

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

$ cxx x.c -emit-ir | mlir-translate -mlir-to-llvmir  |lli --entry-function=to_int; echo $?
1