[llvm] [LoopUnroll] Add `prepare-for-lto` pass option (PR #192153)

[Sean Perry via llvm-commits]

perry-ca wrote:

Looks like bad timing.  It appears the branch I created (https://github.com/llvm/llvm-project/pull/209215) was done between you adding the field and adding this usage.

https://github.com/llvm/llvm-project/pull/192153