Use a more efficient implementation of `ALoc.is_abstract` throughout
Use a more efficient implementation of `ALoc.is_abstract` throughout

Summary: `is_abstract x` is equivalent to `kind x = Abstract`, but it's more efficient. This change significantly decreased the performance regression in D14091208.

Reviewed By: avikchaudhuri

Differential Revision: D15500386

fbshipit-source-id: e5e3a4a7f7a0e144cdbfd1b2e7589fa4d587273a

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