Auto merge of #133734 - scottmcm:lower-indexing-to-ptrmetadata, r=davidtwco,RalfJung

Bounds-check with PtrMetadata instead of Len in MIR

Rather than emitting `Len(*_n)` in array index bounds checks, emit `PtrMetadata(copy _n)` instead -- with some asterisks for arrays and `&mut` that need it to be done slightly differently.

We're getting pretty close to removing `Len` entirely, actually.  I think just one more PR after this (for slice drop shims).

r? mir

Author: bors