Support repeated machine outlining

Summary: The following change is to allow the machine outlining can be applied for Nth times, where N is specified by the compiler option. By default the value of N is 1. The motivation is that the repeated machine outlining can further reduce code size.  Please refer to the presentation "Improving Swift Binary Size via Link Time Optimization" in LLVM Developers' Meeting in 2019.

Reviewers: aschwaighofer, tellenbach, paquette

Reviewed By: paquette

Subscribers: tellenbach, hiraditya, llvm-commits, jinlin

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71027