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

[Jiahan Xie via llvm-commits]

jiahanxie353 wrote:

> Seems like [selectImpl](https://github.com/llvm/llvm-project/blob/e1451236a0a07f1ee4ba5fe3ae2464a82a37c25c/llvm/lib/Target/RISCV/GISel/RISCVInstructionSelector.cpp#L553) can capture loads/stores,

I was wrong, `selectImpl` only supports `rv32` for loading [this example](https://github.com/llvm/llvm-project/blob/907dbb3d9c8dc28590abdf1ff9a808ea64789fae/llvm/test/CodeGen/RISCV/GlobalISel/instruction-select/rvv/load.mir#L232) but not `rv64`.. it seems pretty weird to me since [the table](https://github.com/llvm/llvm-project/blob/75d6795e420274346b14aca8b6bd49bfe6030eeb/llvm/lib/Target/RISCV/RISCVRegisterInfo.td#L268) tells us it should work with `rv64` also, right?

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