Remove special-casing for argument patterns in MIR typeck (attempt to fix perf regression of #133858)
See [my comment](https://github.com/rust-lang/rust/pull/133858#issuecomment-2579029618) on #133858 for more information. This is just a guess as to what went wrong, and I haven't been able to get the profiler running locally, so I'll need a perf run to make sure this actually helps.
There's one test's stderr that suffers a bit, but this was just papering over the issue anyway. Making region errors point to the correct constraints in the presence of invariance/contravariance is a broader problem; the current way it's handled is mostly based on guesswork, luck, and hoping it works out. Properly handling that (somehow) would improve the test's stderr without the hack that this PR reverts.
b44e14f762