[VPlan] Support arbitrary predicated early exits. (#182396)

This removes the restriction requiring a single predicated early exit.
Using MaskedCond, we only combine early-exit conditions with block
masks from non-exiting control flow.

This means we have to ensure that we check the early exit conditions in
program order, to make sure we take the first exit in program order that
exits at the first lane for the combined exit condition.

To do so, sort the exits by their reverse post-order numbers.

Depends on https://github.com/llvm/llvm-project/pull/182395

PR: https://github.com/llvm/llvm-project/pull/182396
GitOrigin-RevId: 320220e48b8f1054ce7a37c8b2a528315080e29c