[Target][RISCV] Add HwMode support to subregister index size/offset. (#86368)

This is needed to provide proper size and offset for the GPRPair subreg
indices on RISC-V. The size of a GPR already uses HwMode. Previously we
said the subreg indices have unknown size and offset, but this stops
DwarfExpression::addMachineReg from being able to find the registers
that make up the pair.

I believe this fixes https://github.com/llvm/llvm-project/issues/85864
but need to verify.