There's a few problems here. The obvious one is that the subreg expression is being thrown away. The not-so obvious one is that once that is fixed, we end up with an invalid instruction:

insn does not satisfy its constraints:
(insn 9 8 11 2 tursi5.c:6 (set (reg:QI 1 r1 [orig:21 D.1197 ] [21])
        (plus:QI (subreg:QI (reg:HI 1 r1) 1)
            (const_int 48 [0x30]))) 59 {addqi3} (nil))

What we need to do is split this into two expressions:
(set (reg:QI 1 r1)
        (subreg:QI (reg:HI 1 r1) 1))
(set (reg:QI 1 r1)
        (plus:QI 1 r1)
            (const_int 48 [0x30]))) 59 {addqi3} (nil))

No problem, right? Well, maybe.

We also need to factor in the live-ness of the register. If in this case we had R2 as the output, we might have to find a temp register to not modify R1. Alternately, we might have to swap R1, do some work and swap it back. Terrible.

On top of that, we might have to implement instructions with every possible variation of arguments. This will quadruple the machine description and make a testing nightmare.

One way out of this particular problem is to make sure that there is a defined mode for every operation. Hopefully, that will minimize the number of these problems. It's just a hunch, but it's the best idea I've got.