Replace calls to is_a with instanceof at IR gen time

Summary: These are equivalent in behavior; the PHP docs for is_a actually say you
should use instanceof instead.

Reviewed By: @edwinsmith

Differential Revision: D1556806
convert mbstring to hni

Summary: fixes #3675

Reviewed By: @ptarjan

Differential Revision: D1543361
Support zlib.output_compression_level option in set_ini

Summary: See previous discussion in post https://fburl.com/48881545 and D1547385, we want to experiment with different gzip compression level to see the impact on client.

Cowardly defered implementation for other zlib options as I'm noob to hhvm.

Reviewed By: @markw65

Differential Revision: D1558813