Auto merge of #151337 - the8472:bail-before-memcpy2, r=Mark-Simulacrum

optimize `vec.extend(slice.to_vec())`, take 2

Redoing rust-lang/rust#130998
It was reverted in rust-lang/rust#151150 due to flakiness. I have traced this to layout randomization perturbing the test (the failure reproduces locally with layout randomization), which is now excluded.

Author: bors