Auto merge of #116047 - a-lafrance:I80836-codegen-test, r=Mark-Simula…

…crum

Add codegen test to guard against VecDeque optimization regression

Very small PR that adds a codegen test to guard against regression for the `VecDeque` optimization addressed in #80836. Ensures that Rustc optimizes away the panic when unwrapping the result of `.get(0)` because of the `!is_empty()` condition.