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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@125748 91177308-0d34-0410-b5e6-96231b3b80d8