Rollup merge of #118688 - celinval:smir-rvalue-ty, r=compiler-errors

Add method to get type of an Rvalue in StableMIR

Provide a method to StableMIR users to retrieve the type of an Rvalue operation. There were two possible implementation:

1. Create the logic inside stable_mir to process the type according to the Rvalue semantics, which duplicates the logic of `rustc_middle::mir::Rvalue::ty()`.
2. Implement the Rvalue translation from StableMIR back to internal representation, invoke the `rustc_middle::mir::Rvalue::ty()`, and translate the return value to StableMIR.

I chose the first one for now since the duplication was fairly small, and the option 2 would require way more work to translate everything back to rustc internal representation. If we eventually add those translations, we could easily swap to the option 2.

```@compiler-errors``` / ```@ouz-a``` Please let me know if you have any strong opinion here.

r? ```@compiler-errors```

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