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

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

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

I have added a TODO in the legalizer to make illegal types of G_IMPLICIT_DEF into legal types, and I have updated this PR to specify that this patch only legalizes vector types that are considered legal.

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