[IndVars] Set Changed if rewriteFirstIterationLoopExitValues changes IR. PR38863

Currently, `rewriteFirstIterationLoopExitValues` does not set Changed flag even if it makes
changes in the IR. There is no clear evidence that it can cause a crash, but it
looks highly suspicious and likely invalid.

Differential Revision: https://reviews.llvm.org/D51779
Reviewed By: skatkov

llvm-svn: 341779