[llvm] [NFC][RISCV] Keep AVLReg define instr inside VSETVLInfo (PR #89180)

[Luke Lau via llvm-commits]

https://github.com/lukel97 approved this pull request.

LGTM. Hopefully then in #70549 we can replace the AVLDef struct with VNInfo or whatever is needed to track definitions post-SSA.

https://github.com/llvm/llvm-project/pull/89180