[VPlan] Add getLiveInIRValue accessor to VPValue.

This patch adds a new getLiveInIRValue accessor to VPValue, which
returns the underlying value, if the VPValue is defined outside of
VPlan. This is required to handle scalars in VPTransformState, which
requires dealing with scalars defined outside of VPlan.

We can simply check VPValue::Def to determine if the value is defined
inside a VPlan.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D92281