[Pipeline] Don't run EarlyFPM in LTO post link

EarlyFPM cleans up the output of the frontend. This isn't necessary in post link pipelines as the pre link pipeline already ran this.

~0.4% savings in ThinLTO builds:
https://llvm-compile-time-tracker.com/compare.php?from=8a5d4eb775c644d8683f24817d44c510d2b853b7&to=3580252a2162eadca0da99f1eeaa112f74a0353d&stat=instructions:u

Reviewed By: tejohnson

Differential Revision: https://reviews.llvm.org/D145403