[RISCV][GISel] Rename PartialMappingIdx and ValueMappingsIdx to use register...

[RISCV][GISel] Rename PartialMappingIdx and ValueMappingsIdx to use register bank name with B suffix. NFC

PartialMappingIdx refers to an entry in the PartMappings table that
includes a RegBank and a size. So I think the PartialMappingIdx should
be named using the name of the register bank concatenated with the size.

So the indices are now PMI_GPRB32, PMI_GPRB64, PMI_FPRB32, PMI_FPRB64.