Fix wrong logic in promotion of signed mul-with-overflow (I pointed this out at

the time but presumably my email got lost).  Examples where the previous logic
got it wrong: (1) a signed i8 multiply of 64 by 2 overflows, but the high part is
zero; (2) a signed i8 multiple of -128 by 2 overflows, but the high part is all
ones. 

llvm-svn: 125748