[llvm] [RISCV][GISEL] Legalize and regbankselect vector typed G_IMPLICIT_DEF (PR #84553)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

Did you try legalization of illegal types, e.g.,  LLT::scalable_vector(30, s1)?

https://github.com/llvm/llvm-project/pull/84553