[SimplifyIndVar] Extract out invariant expression handling

Previously, the code returned early from the *function* when it couldn't find a free expansion, it should be returning from the *transform*.  I don't have a test case, noticed this via inspection.

As a follow up, I'm going to revisit the logic in the extract function.  I think that essentially the whole helper routine can be replaced with SCEVExpander, but I wanted to do that in a series of separate commits.

llvm-svn: 316974