Validate CopyForDeref and DerefTemps better and remove them from runtime MIR

[beepster4096]

(split from my WIP #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 [WIP] Underefer refactoring #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 Custom MIR: Support Rvalue::CopyForDeref #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 DerefTemps 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

[rustbot]

oli-obk is not on the review rotation at the moment.
They may take a while to respond.

[rustbot]

This PR changes MIR

cc @oli-obk, @RalfJung, @JakobDegen, @davidtwco, @vakaras

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

Some changes occurred to constck

cc @fee1-dead

Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

[WaffleLapkin]

(note that oli is on vacation and will be for quite a while)

[beepster4096]

@rustbot author
r? ghost

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[beepster4096]

r? mir
@rustbot ready

[davidtwco]

It's been long enough since I've looked at a involved MIR change that I'm probably not best suited to this

r? mir