[LV] Move recurrence backedge fixup code to VPlan::execute (NFC).

As suggested in D105008, move the code that fixes up the backedge value
for first order recurrences to VPlan::execute.

Now all that remains in fixFirstOrderRecurrences is the code responsible
for creating the exit values in the middle block.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D106244