[RISCV] Switch vwsll.v* instructions to use the VSHT_IV_V_X_I class instead of VALU_IV_V_X_I.

This allows us to remove the uimm5 argument and changes the
scheduler class from ALU to Shift.

Ultimately we need a WShift scheduler class, but we need to scrub
all of the crypto instructions for scheduler classes so I'll leave
that for future work.

Reviewed By: 4vtomat, ego

Differential Revision: https://reviews.llvm.org/D152030