[IndVars][NFC] Refactor to make modifications of Changed transparent

IndVarSimplify's design is somewhat odd in the way how it reports that
some transform has made a change. It has a `Changed` field which can
be set from within any function, which makes it hard to track whether or
not it was set properly after a transform was made. It leads to oversights
in setting this flag where needed, see example in PR38855.

This patch removes the `Changed` field, turns it into a local and unifies
the signatures of all relevant transform functions to return boolean value
which designates whether or not this transform has made a change.

Differential Revision: https://reviews.llvm.org/D51850
Reviewed By: skatkov

llvm-svn: 341893