[RISCV] Support SB/SH/SW in hasAllWUsers in RISCVSExtWRemoval.

After D137446 we can see which operand is the user. If the user
is the value operand of a SB/SH/SW then the upper 32 bits aren't
used.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D137448