Reapply "[RISCV][LoopVectorize] Use DataWithEVL as the preferred tail folding style (#148686)"

This reverts commit 25e97fc420f8ecc43fbabadfe9767b4163e6ee36.

The original commit was reverted due to a crash in llvm-test-suite. The
crash stemmed from a multiply reduction, which isn't supported for
scalable VFs on RISC-V. But for EVL tail folding we only support
scalable VFs, so when -force-tail-folding-style=data-with-evl is
specified we check to see if there's a scalable VF, and fall back to
data-without-lane-mask if there isn't.

This is done in setTailFoldingStyles, but previously we were only
checking if the forced tail folding style was legal, not the style
returned by TTI.

This version fixes this by checking the actual computed tail folding
style and not just the forced one, and adds a test for the crash in
llvm/test/Transforms/LoopVectorize/RISCV/low-trip-count.ll

0 files changed, 0 insertions, 0 deletions