[RISCV] Remove hasSideEffects=1 for saturating/fault-only-first instructions

Marking them as `hasSideEffects=1` stops some optimizations.

According to `Target.td`:

> // Does the instruction have side effects that are not captured by any
> // operands of the instruction or other flags?
> bit hasSideEffects = ?;

It seems we don't need to set `hasSideEffects` for vleNff since we have
modelled `vl` as an output operand.

As for saturating instructions, I think that explicit Def/Use list
is kind of side effects captured by any operands of the instruction,
so we don't need to set `hasSideEffects` either. And I have just
investigated AArch64's implementation, they don't set this flag and
don't add `Def` list.

These changes make optimizations like `performCombineVMergeAndVOps`
and MachineCSE possible for these instructions.

As a consequence, `copyprop.mir` can't test what we want to test in
https://reviews.llvm.org/D155140, so we replace `vssra.vi` with a
VCIX instruction (it has side effects).

Reviewers: jacquesguan, topperc, preames, asb, lukel97

Reviewed By: topperc, lukel97

Pull Request: https://github.com/llvm/llvm-project/pull/90049