[llvm] [WIP] Add new pass for converting scalable vector instructions with known VL to fixed length vectors (PR #190860)

[Ryan Buchner via llvm-commits]

bababuck wrote:

Running on `llvm-test-suite` with the changes from #192068 (though from the discourse conversation it seems that @momchil-velikov has made more changes that haven't been pushed yet):
- 117 `vp.stores` that were suitable candidates for conversion (seeds) (previously 182)
- 9 functions which the pass made changes (previously 19)
- 148 total instructions converted from scalable to fixed (previously 379)

I'll go through and figure out the actual path via which these cases are still being generated. Want to see if this pass is the solution for these cases, or if it would be possible to just create the fixed vectors initially.

https://github.com/llvm/llvm-project/pull/190860