[GISEL] Add G_INSERT_SUBVECTOR and G_EXTRACT_SUBVECTOR (#84538)

G_INSERT and G_EXTRACT are not sufficient to use to represent both
INSERT/EXTRACT on a subregister and INSERT/EXTRACT on a vector.

We would like to be able to INSERT/EXTRACT on vectors in cases that
INSERT/EXTRACT on vector subregisters are not sufficient, so we add
these opcodes.

I tried to do a patch where we treated G_EXTRACT as both
G_EXTRACT_SUBVECTOR and G_EXTRACT_SUBREG, but ran into an infinite loop
at this
[point](https://github.com/llvm/llvm-project/blob/8b5b294ec2cf876bc5eb5bd5fcb56ef487e36d60/llvm/lib/Target/RISCV/RISCVISelLowering.cpp#L9932)
in the SDAG equivalent code.