Auto merge of #150265 - scottmcm:vec-less-ubchecks, r=jhpratt

Stop emitting UbChecks on every Vec→Slice

Spotted this in rust-lang/rust#148766's test changes.  It doesn't seem like this ubcheck would catch anything useful; let's see if skipping it helps perf.  (After all, this is inside *every* `[]` on a vec, among other things.)

Author: rust-bors[bot]