[NewGVN] Mark function as changed if we erase instructions.

Currently eliminateInstructions only returns true if any instruction got
replaced. In the test case for this patch, we eliminate the trivially
dead calls, for which eliminateInstructions not do a replacement and the
function is not marked as changed, which is why the inliner crashes
while traversing the call graph.

Alternatively we could also change eliminateInstructions to return true
in case we mark instructions for deletion, but that's slightly more code
and doing it at the place where the replacement happens seems safer.

Fixes PR37517.

Reviewers: davide, mcrosier, efriedma, bjope

Reviewed By: bjope

Differential Revision: https://reviews.llvm.org/D51169

llvm-svn: 341651