[llvm] [RISCV] Decompose single source shuffles (without exact VLEN) (PR #126108)

[Alex Bradbury via llvm-commits]

asb wrote:

This PR appears to have caused a miscompile on the RVA 23 EVL tail folding buildbot (and very possibly for other RISC-V vector configs - I haven't had a chance to explore). The miscompile is exposed through incorrect output from the stage2 compiler. However I've managed to reproduce locally with a single-stage build of Clang, then cross-compiling the llvm-test-suite. `20050826-2.c` from the GCC torture suite is failing but passes if you revert 8374d42 (the commit landed from this PR).

I'm currently double checking that the commit cleanly reverts (i.e. there are no later test changes that would be invalidated by the revert).

https://github.com/llvm/llvm-project/pull/126108