improve optimization of invoke instructions:

 - simplifycfg:  invoke undef/null -> unreachable
 - instcombine:  invoke new  -> invoke expect(0, 0)  (an arbitrary NOOP intrinsic;  only done if the allocated memory is unused, of course)
 - verifier:  allow invoke of intrinsics  (to make the previous step work)

llvm-svn: 159146