[LV] Move code from widenInstruction to VPWidenRecipe. (NFC)

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:

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/D114322