[SDAG] Scalarization as an option in `LegalizeAction`

[natanelh-mobileye]

Sometimes, a target might desire to legalize an operation by scalarizing it.
Consider a target that has a scalar ISD::ABDS for MVT::i16 but no vector version for MVT::v2i16. the current options are quite limited:

  enum LegalizeAction : uint8_t {
    Legal,      // The target natively supports this operation.
    Promote,    // This operation should be executed in a larger type.
    Expand,     // Try to expand this to other ops, otherwise use a libcall.
    LibCall,    // Don't try to expand this to other ops, always use a libcall.
    Custom      // Use the LowerOperation hook to implement custom lowering.
  };

clearly it is not Legal, Promote might not be supported, Expand returns in our case completely different ops (min, max, sub, which is 3 instructions instead of two vector instructions), LibCall is more expensive than two instructions and we are left with Custom - in a lot of cases we just point to one function downstream that basically scalarizes the operation or converts to shorter vectors (v16i16 -> v8i16 x 2). Could be great to have that upstream. opinions?

(that or just "try reduce the type, then combine immediately/call hook" instead)

or maybe consider adding a utility function that does it,