Use musttail to enforce tail call.

Summary: From LLVM manual: "The musttail marker means that the call must be tail call optimized in order for the program to be correct". We should use it when we need tail call to happen.

Reviewed By: @jasone

Differential Revision: D1682217

Signature: t1:1682217:1415995606:133db7fff792a61fb7c199febbe4e31eea8d1ea3