[RISCV] Scalarize binop followed by extractelement to custom lowered instruction

isOperationLegalOrCustomOrPromote returns true only if VT is other or legal
and operation action is Legal, Custom or Promote.
Permit a vector binary operation can be converted to scalar binary operation which is custom lowered with illegal type.
One of cases is i32 isn't a legal type on RV64 and its ALU operations is set to custom lowering,
so vadd for element type i32 can be converted to addw.

Reviewed By: jacquesguan, craig.topper

Differential Revision: https://reviews.llvm.org/D156692