[llvm] [RISCV][GISEL] Legalize and post-legalize lower G_INSERT_SUBVECTOR (PR #108859)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

What is the difference between hosting this code at:
* the legalizer
* the post legalizer lower combiner
* the instruction selector?

https://github.com/llvm/llvm-project/pull/108859