Make check_match not copy the AST

After #13316 check_match is a major bottleneck in the cycle time of rustc taking 4 times more time than it did before (18-19 s vs 4.5 s).

The main problem is the static inliner which creates new AST nodes. It should be possible to make it not do that.

[eddyb]

Have you checked after I removed the clone call which resulted in short-lived copies amounting to 2-3 times the crate size?

I hear it's miraculously back to 4s. The bug is still valid, though.

[steveklabnik]

This is an old bug, and the original reporter is gone, and the compiler has gone through a ton of stuff, so just closing.