[x86] Fix PR20355 (and dups) by not using unsigned multiplication when
signed multiplication is requested. While there is not a difference in
the *low* half of the result, the *high* half (used specifically to
implement the signed division by these constants) certainly is used. The
test case I've nuked was actively asserting wrong code.

There is a delightful solution to doing signed multiplication even when
we don't have it that Richard Smith has crafted, but I'll add the
machinery back and implement that in a follow-up patch. This at least
restores correctness.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214007 91177308-0d34-0410-b5e6-96231b3b80d8
[x86] Revert r214007: Fix PR20355 ...

The clever way to implement signed multiplication with unsigned *is
already implemented* and tested and working correctly. The bug is
somewhere else. Re-investigating.

This will teach me to not scroll far enough to read the code that did
what I thought needed to be done.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214009 91177308-0d34-0410-b5e6-96231b3b80d8