Auto merge of #126541 - scottmcm:more-ptr-metadata-gvn, r=cjgillot

More ptr metadata gvn

There's basically 3 parts to this PR.

1. Allow references as arguments to `UnOp::PtrMetadata`

This is a MIR semantics addition, so
r? mir

Rather than just raw pointers, also allow references to be passed to `PtrMetadata`.  That means the length of a slice can be just `PtrMetadata(_1)` instead of also needing a ref-to-pointer statement (`_2 = &raw *_1` + `PtrMetadata(_2)`).

AFAIK there should be no provenance or tagging implications of looking at the *metadata* of a pointer, and the code in the backends actually already supported it (other than a debug assert, given that they don't care about ptr vs reference, really), so we might as well allow it.

2. Simplify the argument to `PtrMetadata` in GVN

Because the specific kind of pointer-like thing isn't that important, GVN can simplify all those details away.  Things like `*const`-to-`*mut` casts and `&mut`-to-`&` reborrows are irrelevant, and skipping them lets it see more interesting things.

cc `@cjgillot`

Notably, unsizing casts for arrays.  GVN supported that for `Len`, and now it sees it for `PtrMetadata` as well, allowing `PtrMetadata(pointer)` to become a constant if that pointer came from an array-to-slice unsizing, even through a bunch of other possible steps.

3. Replace `NormalizeArrayLen` with GVN

The `NormalizeArrayLen` pass hasn't been running even in optimized builds for well over a year, and it turns out that GVN -- which *is* on in optimized builds -- can do everything it was trying to do.

So the code for the pass is deleted, but the tests are kept, just changed to the different pass.

As part of this, `LowerSliceLen` was changed to emit `PtrMetadata(_1)` instead of `Len(*_1)`, a small step on the road to eventually eliminating `Rvalue::Len`.

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