[x86] Make the retpoline thunk insertion a machine function pass.

Summary:
This removes the need for a machine module pass using some deeply
questionable hacks. This should address PR36123 which is a case where in
full LTO the memory usage of a machine module pass actually ended up
being significant.

We should revert this on trunk as soon as we understand and fix the
memory usage issue, but we should include this in any backports of
retpolines themselves.

Reviewers: echristo, MatzeB

Subscribers: sanjoy, mcrosier, mehdi_amini, hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D42726

llvm-svn: 323915