[LV] Move reduction PHI node fixup to VPlan::execute (NFC).

All information to fix-up the reduction phi nodes in the vectorized loop
is available in VPlan now. This patch moves the code to do so, to make
this clearer. Fixing up the loop exit value still relies on other
information and remains outside of VPlan for now.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D100113