[IPSCCP] Add assertion to surface cases where we zap returns with overdefined users.

We should only zap returns in functions, where all live users have a
replace-able value (are not overdefined). Unused return values should be
undefined.

This should make it easier to detect bugs like in PR42738.

Alternatively we could bail out of zapping the function returns, but I
think it would be better to address those divergences between function
and call-site values where they are actually caused.

Reviewers: davide, efriedma

Reviewed By: davide, efriedma

Differential Revision: https://reviews.llvm.org/D65222

llvm-svn: 366998