change instcombine to not turn a call to non-varargs bitcast of

change instcombine to not turn a call to non-varargs bitcast of
function prototype into a call to a varargs prototype.  We do
allow the xform if we have a definition, but otherwise we don't
want to risk that we're changing the abi in a subtle way.  On
X86-64, for example, varargs require passing stuff in %al.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@126363 91177308-0d34-0410-b5e6-96231b3b80d8