Improve the widening of integral binary vector operations

- split WidenVecRes_Binary into WidenVecRes_Binary and WidenVecRes_BinaryCanTrap
  - WidenVecRes_BinaryCanTrap preserves the original behaviour for operations
    that can trap
  - WidenVecRes_Binary simply widens the operation and improves codegen for
    3-element vectors by allowing widening and promotion on x86 (matches the
    behaviour of unary and ternary operation widening)
- use WidenVecRes_Binary for operations on integers.

Reviewed by: nrotem

llvm-svn: 188699