First iteration of AllocaOp, LoadOp and StoreOp

[robertoraggi]

They are missing traits, custom builders and
assembly labels.

Fixes #616

void f() {}

int main(int argc, char** argv) {}

$ cxx x.c -emit-ir

!i32s = !cxx.int<32, true>
module @x.c {
  cxx.func @f() {
    cf.br ^bb1
  ^bb1:  // pred: ^bb0
    "cxx.return"() : () -> ()
  }
  cxx.func @main(%arg0: !i32s, %arg1: !cxx.ptr<!cxx.ptr<!cxx.expr>>) -> !i32s {
    %0 = "cxx.alloca"() : () -> !cxx.ptr<!i32s>
    cf.br ^bb1(%0 : !cxx.ptr<!i32s>)
  ^bb1(%1: !cxx.ptr<!i32s>):  // pred: ^bb0
    %2 = "cxx.load"(%0) : (!cxx.ptr<!i32s>) -> !i32s
    "cxx.return"(%2) : (!i32s) -> ()
  }
}