Auto merge of #116826 - nnethercote:fix-116780-116797, r=compiler-errors

Fix a performance regression in obligation deduplication.

Commit 8378487 from #114611 changed the location of an obligation deduplication step in `opt_normalize_projection_type`. This meant that deduplication stopped happening on one path where it was still necessary, causing a couple of drastic performance regressions.

This commit moves the deduplication back to the old location. The good news is that #114611 had four commits and 8378487 was of minimal importance, so the perf benefits from that PR remain.

Fixes #116780, #116797.

r? `@compiler-errors`