[RISCV] Set how many bytes load from or store to stack slot

Refer from: https://reviews.llvm.org/D44782

After https://reviews.llvm.org/D130302, LW+SEXT.B can be folded into LB
as partially reload stack slot. This gains incorrect optimization result
from `StackSlotColoring` without given the number of bytes exactly load
from stack. LB+SW are mis-interpreted as fully reload/restore from stack
slot without the sign-extension. SW would be considered as a redundant store.

The testcase is copied from llvm/test/CodeGen/X86/pr30821.mir.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D145471