[RISCV] Add i8/i16 test cases to div.ll and i8/i16/i64 to rem.ll. NFC

This improves our coverage of these operations and shows that we
use really large constants for division by constant on i8/i16
especially on RV64. The issue is that BuildSDIV/BuildUDIV are
limited to legal types so we have to promote to i64 before it
kicks in. At that point we've lost the range information for the
original type.