Relands this feature after several fixes:
* Force fake uses to be emitted before musttail calls (#136867)
* Added soften-float legalization for fake uses (#142714)
* Treat fake uses as size-less instructions in a SystemZ assert (#144390)
If further issues with fake uses are found then this may be reverted again,
but all currently-known issues are resolved.
This reverts commit 2dc6e98169baeb1f73036da0ea50fd828d8323d0.
36af7345df