[LV][RISCV] Disable vectorization of epilogue loops

[LV][RISCV] Disable vectorization of epilogue loops

Epilogue loop vectorization is a feature in the vectorize intended to avoid running fully scalar code when the vector length of the main loop turns out to be either longer than the trip count of the actual loop, or with a huge remainder.

In practice, this feature appears to not have been well tuned. I honestly don't think it should be on by default at all, but it definitely shouldn't be on for RISCV. Note that other targets have also disabled it, but they've done so via disabling interleaving - which is, well, completely unrelated - and we don't want to do that for RISCV.

In the near term, many examples I'm seeing have terrible codegen for epilogue vectorization. We are greatly increasing code size for little value at reasonable VLEN values for small types. In the long term, the cases that epilogue vectorization are intended to handle are likely better handled via tail folding on RISCV.

As an aside, I also don't really trust the correctness of epilogue vectorization. The code structure is such that otherwise straight forward changes sometimes break only epilogue vectorization. The reuse of an existing vplan without careful validation opens significant room for nasty bugs. Given how rarely the code is exercised, that is not a good combination.

As such, this patch introduces a TTI hook, and completely disables epilogue vectorization on RISCV.

Differential Revision: https://reviews.llvm.org/D136695