[llvm] [RISCV][GISEL] Legalize G_INSERT_SUBVECTOR (PR #108859)

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

> How does one create a G_INSERT_SUBVECTOR today? There are no callers of `MachineIRBuilder::buildInsertSubvector`

It will be needed by G_INSERT_VECTOR_ELT. This is a precursor to that patch.

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