[RISCV] Add i8 and i16 srli and srai tests to Zbb/Zbp test files. NFC

These require the input to be zero or sign extended. If we have
sext.b, sext.h or zext.h instructions we can use them. Otherwise
we need to use a pair of shifts to accomplish the zero/sign extend
and the final shift.

We don't currently use zext.h when it is available.