[llvm] [RISCV] Prefer VLS over VLA if costs are equal (PR #100564)

[Philip Reames via llvm-commits]

preames wrote:

I would prefer this not land without some evidence of profitability on at least some hardware.

I glanced at something like this - though I keyed mine off having an exactly known VLEN from -mrvv-vector-bits=zvl - and glanced at some test deltas.  I didn't see much, and what I did see appeared to be easily addressable in the scalable lowering.  

I'm not really opposed to the idea, I just want to make sure we're not carrying complexity for no value.  

https://github.com/llvm/llvm-project/pull/100564