[ARM] Prevent parallel macs for unsigned values

Both zext and sext are currently allowed during the search for narrow
sequences and sexts operands are later added to the mac candidates.
But operands of muls are also added, without checking whether they're
sext or zext, which means we can generate a signed smlad when we
shouldn't.

Differential Revision: https://reviews.llvm.org/D54790

llvm-svn: 347542