[LFTR] Hoist extend expressions outside of loops w/o waiting for LICM

The motivation for this is two fold:
1) Make the output (and thus tests)  a bit more readable to a human trying to understand the result of the transform
2) Reduce spurious diffs in a potential future change to restructure all of this logic to use SCEVExpander (which hoists by default)

llvm-svn: 365066