[WebAssembly] [Backend] Wasm optimize illegal bitmask for #131980.
Currently, the case for illegal bitmask (v32i8 or v64i8) is that at the
SelectionDag level, two (four) vectors of v128 will be concatenated
together, then they'll all be SETCC by the same pseudo illegal
instruction, which requires expansion later on.
I opt for SETCC-ing them seperately, bitcast and zext them and then add
them up together in the end.
---------
Co-authored-by: badumbatish <--show-origin>
Correctly handle single-element vectors to fix an assertion failure. Add tests
that were missing from the original commit.
Differential Revision: D151782
