Revert "Enable most optimizations with loops"

Summary: This reverts commit 9f0c92ba1c1e9de9266b896d19874f45be5da325.

Was breaking hphp_dbgo_jit_loops run.

Reviewed By: @bertmaher, @ptarjan

Differential Revision: D1493916
Implement `idx` gdb command

Summary: Much like the corresponding PHP operation, `idx` is intended to take an
arbitrary container and an index and return container[index].  Currently
it only operates on vectors and generic objects, but support for more
containers is coming soon.

Reviewed By: @alexmalyshev

Differential Revision: D1493896