remove trigger_error from sncompress

Summary: These are hilariously causing a 20% perf regression. I'll put them back once I can migrate us.

Reviewed By: @JoelMarcey

Differential Revision: D1462853
Fix codegen bug when Class* doesn't fit in a dword

Summary: We were assuming that Class* fits in 32-bits, but that's not
necessarily the case.  If it doesn't fit, fall back to using a scratch
register.

Reviewed By: @markw65

Differential Revision: D1465175
make DynamicExtensions work in ini

Summary: I need to change all the callsites, but this one has been asked for the most.

Reviewed By: @fredemmott

Differential Revision: D1464698