Several changes to make simple tracelets pass with LLVM code gen.

Summary: Use HHVM_TC calling convention to gain access to %rbx, %r12, %rbp.

Workaround for non-SSA reserved registers.

Implementation for direct vcall/vinvoke instructions.

Substitution of pushm/ret combo with a tail call.

Fake personality function recognizable by LLVM runtime.

Emit set of instructions for minimal tracelets to pass.

Force no function alignment for emitted code.

Override stack alignment with 8 bytes.

Added a couple more optimization passes.

Create own MInstrState area and redirect %rsp-relative addresses to it.

Reviewed By: @swtaarrs

Differential Revision: D1616966