[LV] Move code from widenSelectInstruction to VPWidenSelectRecipe. (NFC)

The code in widenSelectInstruction has already been transitioned
to only rely on information provided by VPWidenSelectRecipe directly.

Moving the code directly to VPWidenSelectRecipe::execute completes
the transition for the recipe.

It provides the following advantages:

1. Less indirection, easier to see what's going on.
2. Removes accesses to fields of ILV.

2) in particular ensures that no dependencies on
fields in ILV for vector code generation are re-introduced.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D114323