builders: Assume abstract registers
builders: Assume abstract registers

Summary:
See the summary of {D6241960}

Note that this diff does not cause remove-builders to generate
non-register-width-respecting bytecode. If the input bytecode does give wide
registesr two IDs, the output bytecode will still respect this invariant. This
diff just simplifies our analysis, and allows remove-builders to keep working
even if the input does not respect register widths. This allows for a gradual
migration plan.

Once all passes can accept abstract registers, remove-builders can be further
simplified.

Reviewed By: arnaudvenet

Differential Revision: D6242637

fbshipit-source-id: 3218e52d942806b45eb776677d6ff9c65bc957ec

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