[VPlan] Use Exiting-block instead of Exit-block terminology (NFC).

In LLVM's common loop terminology, an exit block is a block outside a
loop with a predecessor inside the loop. An exiting block is a block
inside the loop which branches to an exit block outside the loop.

This patch updates a few places where VPlan was using ExitBlock for a
block exiting a region. Those instances have been updated to use
ExitingBlock.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D126173