[LV] Move code from widenGEP to VPWidenGEPRecipe (NFC).

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

Moving the code directly to VPWidenGEPRecipe::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 GEP code generation are re-introduced.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D114321