Fix a pair of issues that caused an infinite loop in reassociate.

Terrifyingly, one of them is a mishandling of floating point vectors
in Constant::isZero().  How exactly this issue survived this long
is beyond me.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@253655 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed