Rewrite calls to bitcast unprototyped functions when emitting a definition.

My variadics patch, r169588, changed these calls to typically be
bitcasts rather than calls to a supposedly variadic function.
This totally subverted a hack where we intentionally dropped
excess arguments from such calls in order to appease the inliner
and a "warning" from the optimizer.  This patch extends the hack
to also work with bitcasts, as well as teaching it to rewrite
invokes.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@170034 91177308-0d34-0410-b5e6-96231b3b80d8