[LoopVectorize] Use new getUniqueLatchExitBlock routine (#108231) With PR #88385 I am introducing support for vectorising more loops with early exits that don't require a scalar epilogue. As such, if a loop doesn't have a unique exit block it will not automatically imply we require a scalar epilogue. Also, in all places in the code today where we use the variable LoopExitBlock we actually mean the exit block from the latch. Therefore, it seemed reasonable to add a new getUniqueLatchExitBlock that allows the caller to determine the exit block taken from the latch and use this instead of getUniqueExitBlock. I also renamed LoopExitBlock to be LatchExitBlock. I feel this not only better reflects how the variable is used today, but also prepares the code for PR #88385. While doing this I also noticed that one of the comments in requiresScalarEpilogue is wrong when we require a scalar epilogue, i.e. when we're not exiting from the latch block. This doesn't always imply we have multiple exits, e.g. see the test in Transforms/LoopVectorize/unroll_nonlatch.ll where the latch unconditionally branches back to the only exiting block.
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