[FIRRTL][Ref] Determine rules about ref.send under when

[dtzSiFive]

It's strange/nonsensical for the anchor of the dataflow to be conditional (send), it should be unconditional. resolve does not have this issue.

Example:

firrtl.circuit "xmr" {
  firrtl.module private @Test(in %p: !firrtl.uint<1>, out %x: !firrtl.ref<uint<2>>) {
    firrtl.when %p {
      %zero = firrtl.constant 1 : !firrtl.uint<2>
      %w = firrtl.wire : !firrtl.uint<2>
      %1 = firrtl.ref.send %w : !firrtl.uint<2>
      firrtl.strictconnect %x, %1 : !firrtl.ref<uint<2>>
      firrtl.strictconnect %w, %zero : !firrtl.uint<2>
    }
  }
  firrtl.module @xmr(in %p: !firrtl.uint<1>) {
    %test_in, %test_x = firrtl.instance test @Test(in p: !firrtl.uint<1>, out x: !firrtl.ref<uint<2>>)
    firrtl.strictconnect %test_in, %p : !firrtl.uint<1>
    %x = firrtl.ref.resolve %test_x : !firrtl.ref<uint<2>>
  }
}

A send from within a when should work if the resolve is also in the same when block.

EDIT: previously suggested this should be illegal, I think the above example is okay, if needs some work to make that seem natural. Only one connect to the output ref port is allowed (already checked), but the above example is rejected due to not fully initializing %x.