Auto merge of #145513 - beepster4096:erasedereftemps, r=saethlin,cjgillot

Validate CopyForDeref and DerefTemps better and remove them from runtime MIR

(split from my WIP rust-lang/rust#145344)

This PR:
- Removes `Rvalue::CopyForDeref` and `LocalInfo::DerefTemp` from runtime MIR
    - Using a new mir pass `EraseDerefTemps`
    - `CopyForDeref(x)` is turned into `Use(Copy(x))`
    - `DerefTemp` is turned into `Boring`
        - Not sure if this part is actually necessary, it made more sense in rust-lang/rust#145344 with `DerefTemp` storing actual data that I wanted to keep from having to be kept in sync with the rest of the body in runtime MIR
- Checks in validation that `CopyForDeref` and `DerefTemp` are only used together
- Removes special handling for `CopyForDeref` from many places
- Removes `CopyForDeref` from `custom_mir` reverting rust-lang/rust#111587
    - In runtime MIR simple copies can be used instead
    - In post cleanup analysis MIR it was already wrong to use due to the lack of support for creating `DerefTemp` locals
    - Possibly this should be its own PR?
 - Adds an argument to `deref_finder` to avoid creating new `DerefTemp`s and `CopyForDeref` in runtime MIR.
     - Ideally we would just avoid making intermediate derefs instead of fixing it at the end of a pass / during shim building
 - Removes some usages of `deref_finder` that I found out don't actually do anything

r? oli-obk

compiler/rustc_mir_transform/src/ref_prop.rs

@@ -247,8 +247,7 @@ fn compute_replacement<'tcx>(
             // This is a copy, just use the value we have in store for the previous one.
             // As we are visiting in `assignment_order`, ie. reverse postorder, `rhs` should
             // have been visited before.
-            Rvalue::Use(Operand::Copy(place) | Operand::Move(place))
-            | Rvalue::CopyForDeref(place) => {
+            Rvalue::Use(Operand::Copy(place) | Operand::Move(place)) => {
                 if let Some(rhs) = place.as_local()
                     && ssa.is_ssa(rhs)
                 {