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

[Thorsten Schütt via llvm-commits]

tschuett wrote:

Disallow illegal types! But the legalizer should transform illegal types into legal types. For scalable vectors there is not yet good test coverage.

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