[HotColdSplit] Allow outlining with live outputs

Prior to r348205, extracting code regions with live output values was
disabled because of a miscompilation (PR39433). Lift the restriction as
PR39433 has been addressed.

Tested on LNT+externals, on a run of check-llvm in a stage2 build, and
with a full build of iOS (with hot/cold splitting enabled).

As a drive-by, remove an errant TODO.

llvm-svn: 351492