[LV] Pick correct BB as insert point when fixing PHI for FORs.

Currently we fail to pick the right insertion point when
PreviousLastPart of a first-order-recurrence is a PHI node not in the
LoopVectorBody. This can happen when PreviousLastPart is produce in a
predicated block. In that case, we should pick the insertion point in
the BB the PHI is in.

Fixes PR44020.

Reviewers: hsaito, fhahn, Ayal, dorit

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D71071