[llvm] [RISC-V][GISEL] Select G_BITCAST for scalable vectors (PR #101486)

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

According to [RISCVDAGToADAGISel::Select](https://github.com/llvm/llvm-project/blob/130c135689ec12ab78c53645808524a8d28f7cae/llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp#L2170), we should drop bitcasts between vectors if both are fixed or both are scalable.

https://github.com/llvm/llvm-project/pull/101486