Auto merge of #136771 - scottmcm:poke-slice-iter-next, r=joboet

Simplify `slice::Iter::next` enough that it inlines

Inspired by this zulip conversation: <https://rust-lang.zulipchat.com/#narrow/channel/189540-t-compiler.2Fwg-mir-opt/topic/Feedback.20on.20a.20MIR.20optimization.20idea/near/498579990>

~~Draft for now because it needs #136735 to get the codegen tests to pass.~~

Author: bors