[RISCV][GISel] Make register bank selection for unary and binary arithmetic...

[RISCV][GISel] Make register bank selection for unary and binary arithmetic ops more generic. (#87593)

This is inspired by AArch64's getSameKindOfOperandsMapping, but based on
what RISC-V currently needs.

This removes the special vector case for G_ADD/SUB and unifies integer
and FP operations into the same handler.

G_SEXTLOAD/ZEXTLOAD have been separated from integer since they should
only be scalar integer and never vector.