[llvm] [RISCV] Recursively split concat_vector into smaller LMULs (PR #83035)

[Luke Lau via llvm-commits]

lukel97 wrote:

> I'm a bit hesitant about this being a combine instead of a lowering. In particular, I'm concerned about the split form inhibiting other combines - such as e.g. the load combine. 

Yup, looks like this is interfering with the strided load combine on some x264 workloads, e.g. `pixel_var_8x8` in pixel.c. I've reverted this in ef520ca6b18cf65fc7a13fcad1534f1d70332a88

https://github.com/llvm/llvm-project/pull/83035