As noted in the reproducer provided in
https://github.com/llvm/llvm-project/issues/164762#issuecomment-3554719231,
on RISC-V after LTO we sometimes have trip counts exposed to vectorized
loops. The loop vectorizer will have generated calls to
@llvm.experimental.get.vector.length, but there are [some
properties](https://llvm.org/docs/LangRef.html#id2399) about the
intrinsic we can use to simplify it:
- The result is always less than both Count and MaxLanes
- If Count <= MaxLanes, then the result is Count
This teaches SCCP to handle these cases with the intrinsic, which allows
some single-iteration-after-LTO loops to be unfolded.
#169293 is related and also simplifies the intrinsic in InstCombine via
computeKnownBits, but it can't fully remove the loop since
computeKnownBits only does limited reasoning on recurrences.
2c9e9ffa77