[llvm] [RISCV][VLOPT] Add support for vrgather (PR #148249)

[Philip Reames via llvm-commits]

https://github.com/preames commented:

vrgather has a nasty semantic cornercase where we can't reduce the VL of the data operand.  We can reduce the VL of the vrgather itself, and of the index operand, but not the data operand because vrgather can read past VL.  Do you have a test for that case?  I didn't spot it immediately, but might have missed it.  

https://github.com/llvm/llvm-project/pull/148249