Temporarily bring back Rvalue::Len

[lqd]

r? @compiler-errors as requested in #135671 (comment)

However, in the mean time, I'd rather we not crunch trying to find and more importantly validate the soundness of a solution 🤔

Agreed. To fix the IMO P-critical #135671 for which we somehow didn't have test coverage, this PR temporarily reverts:

• Bounds-check with PtrMetadata instead of Len in MIR #133734
• its bugfix Check for array lengths that aren't actually usize #134371
• Delete Rvalue::Len 🎉 #134330

cc @scottmcm

I added the few samples from that issue as a test, but we can add more in the future, in particular it seems @steffahn will work on that.

Fixes #135671. And if we want to land this, it should also be nominated for beta backport.