[llvm] [LV] Enable auto-vectorisation of loops with uncountable exits (PR #133099)

[Luke Lau via llvm-commits]

lukel97 wrote:

I tried this out on RISC-V and we get 107% more loops vectorized on one of the SPEC CPU 2017 benchmarks, with an overall geomean 5% improvement across all the benchmarks.

There's also an LNT run [here](https://lnt.lukelau.me/db_default/v4/nts/369?show_delta=yes&show_previous=yes&show_stddev=yes&show_mad=yes&show_all=yes&show_all_samples=yes&show_sample_counts=yes&show_small_diff=yes&num_comparison_runs=0&test_filter=&test_min_value_filter=&aggregation_fn=min&MW_confidence_lv=0.05&compare_to=370&submit=Update) and there's no regressions. So no issues on the RISC-V side!

https://github.com/llvm/llvm-project/pull/133099