[RISCV]Remove solved TODO for combining constant shifts

[Luhaocong]

Summary

The constant shifts have be combined, so remove TODO.

[asb]

LGTM, thanks.

llvm/test/CodeGen/RISCV/rv64i-exhaustive-w-insts.ll

 ; RV64-LABEL: sext_slliw_zext:
 ; RV64:       # %bb.0:
 ; RV64-NEXT:    slliw a0, a0, 6
 ; RV64-NEXT:    ret
   %1 = shl i32 %a, 6
   ret i32 %1
 }
 
-; TODO: the constant shifts could be combined.
-
 define zeroext i32 @zext_slliw_aext(i32 %a) nounwind {
 ; RV64-LABEL: zext_slliw_aext:
 ; RV64:       # %bb.0:
 ; RV64-NEXT:    slli a0, a0, 39
 ; RV64-NEXT:    srli a0, a0, 32
 ; RV64-NEXT:    ret
   %1 = shl i32 %a, 7
   ret i32 %1