[LV] Vectorize early exit loops with multiple exits. (#174864)

Building on top of the recent changes to introduce BranchOnTwoConds,
this patch adds support for vectorizing loops with multiple early exits,
all dominating a countable latch. The early exits must form a
dominance chain, so we can simply check which early exit has been taken
in dominance order.

Currently LoopVectorizationLegality ensures that all exits other than
the latch must be uncountable. handleUncountableEarlyExits now collects
those uncountable exits and processes each exit.

In the vector region, we compute if any exit has been taken, by taking
the OR of all early exit conditions (EarlyExitConds) and checking if
there's
any active lane.

If the early exit is taken, we exit the loop and compute which early
exit
has been taken. The first taken early exit is the one where its exit
condition is true in the first active lane of EarlyExitConds.

We create a chain of dispatch blocks outside the loop to check this for
the early exit blocks ordered by dominance.

Depends on https://github.com/llvm/llvm-project/pull/174016.

PR: https://github.com/llvm/llvm-project/pull/174864