The code in widenInstruction has already been transitioned to
only rely on information provided by VPWidenRecipe directly.
Moving the code directly to VPWidenRecipe::execute completes
the transition for the recipe.
It provides the following advantages:
- Less indirection, easier to see what's going on.
- Removes accesses to fields of ILV.
- in particular ensures that no dependencies on
fields in ILV for vector code generation are re-introduced.