Improve end of expression check in for loop lints
Improve end of expression check in for loop lints

The code should to check that the current expression _is_ the end
expression; not that it's equal to it. The equality check seems very
wasteful in terms of performance.

graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust