This fixes#2672 and #2679, and also fixes an incorrect instruction
emission (`test` with small immediate) that we had missed earlier.
The shift-related fixes have to do with (i) shifts by 0 bits, as a
special case that must be handled; and (ii) shifts by a 128-bit amount,
which we can handle by just dropping the upper half (we only use 3--7
bits of shift amount).
This adjusts the lowerings appropriately, and also adds run-tests to
ensure that the lowerings actually execute correctly (previously we only
had compile-tests with golden lowerings; I'd like to correct this for
more ops eventually, adding run-tests beyond what the Wasm spec and
frontend covers).
0f3e00b25e