hugr-llvm: Add expect_tag to LLVMSumValue

[doug-q]

We should emit https://llvm.org/docs/LangRef.html#llvm-expect-intrinsic llvm ops when we can, to help the optimiser know where hot paths are.

I suggest we can do this easily by adding

impl LLVMSumValue {
  fn expect_tag(mut self, tag: usize) -> Self {
    self.expect = Some(self.tag_type().const_int(tag, false));
    self
  }

And where expect: Option<IntValue<'c>> is an additional field in LLVMSumValue.

Now when we build_get_tag we insert an expect intrinsic if self.expect.is_some().