[RISCV] Remove G_TRUNC/ZEXT/SEXT/ANYEXT from the first switch in...

[RISCV] Remove G_TRUNC/ZEXT/SEXT/ANYEXT from the first switch in RISCVRegisterBankInfo::getInstrMapping.

This removes the special case for vectors. The default case in the
second switch can handle GPR in addition to vectors. We just won't
use the static ValueMapping entry.