Wasm backend MulMayOflo issue

Reported in a test-primops run:

MulMayOflo
    W8:                   FAIL (25.09s)
      *** Failed! Falsified (after 1 test and 2 shrinks):
      2::W8
      0x4e::W8
      156
      Use --quickcheck-replay=116271 to reproduce.
      Use -p '/MulMayOflo.W8/' to rerun this test only.
    W16:                  FAIL (24.10s)
      *** Failed! Falsified (after 1 test and 1 shrink):
      6::W16
      0x2217::W16
      52362
      Use --quickcheck-replay=531378 to reproduce.
      Use -p '/MulMayOflo.W16/' to rerun this test only.

W32 passes testing, W64 fails from boring function signature mismatch.

Now, GHC only emits platform word sized MulMayOflo when lowering STG to Cmm, so it doesn't sound like a big deal to me. But it seems to be possible to make subword MulMayOflo in hand-written Cmm. What's the semantics in the subword cases? And does anyone actually do that?