[LV] Check if plan has an early exit via plan's exit blocks. (NFC) (#134720)
Add a dedicated function to check if a plan is for a loop with an early
exit. This can easily be determined by checking the exit blocks.
This allows removing a use of Legal->hasUncountableEarlyExit() from
InnerLoopVectorizer.
PR: https://github.com/llvm/llvm-project/pull/134720