[NFCI][IndVars] rewriteLoopExitValues(): don't expand SCEV's until needed

Previously, we'd expand *ALL* the SCEV's eagerly, because we needed to
check with `isValidRewrite()`, and discard bad rewrite candidates,
but now that we do not do that, we also don't need to always expand.

In particular, this avoids expanding potentially-huge SCEV's that we
would discard anyways because they are high-cost and we aren't
rewriting aggressively.