Remove a useless ref/id/ref round-trip from `pattern_from_hir`
This re-lookup of `&hir::Pat` by its ID appears to be an artifact of earlier complexity that has since been removed from the compiler.
Merely deleting the let/match results in borrow errors, but sprinkling `'tcx` in the signature allows it to work again, so I suspect that this code's current function is simply to compensate for overly loose lifetimes in the signature. Perhaps it made more sense at a time when HIR lifetimes were not tied to `'tcx`.
I spotted this while working on some more experimental changes, which is why I've extracted it into its own PR.
4e6147d956