[RISCV] Refactor subreg indices. (#77173)

-Rename sub_32_hi to sub_gpr_odd
-Add dedicated sub_gpr_even.
-Rename sub_32 and sub_16 to sub_fpr32 and sub_fpr16.
-Remove start offset from sub_gpr_odd. AArch64 doesn't use non-zero offset for GPR
tuples so I don't think we need to.

This is preparation for a RV64 GPRPair for Zacas.