Reland 7691790d with a MSAN fix

In some cases when HexagonTargetLowering::allowsMemoryAccess returned
true, it did not set the "Fast" argument, leaving it uninitialized.

[Hexagon] Improve casting of boolean HVX vectors to scalars

- Mark memory access for bool vectors as disallowed in target lowering.
  This will prevent combining bitcasts of bool vectors with stores.
- Replace the actual bitcasting code with a faster version.
- Handle casting of v16i1 to i16.