[DAGCombiner] simplify div/rem-by-0
Refactoring of duplicated code and more fixes to follow.
This is motivated by the post-commit comments for r296699:
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-
20170306/435182.html
Ie, we can crash if we're missing obvious simplifications like this that
exist in the IR simplifier or if these occur later than expected.
The x86 change for non-splat division shows a potential opportunity to improve
vector codegen: we assumed that since only one lane had meaningful results, we
should do the math in scalar. But that means moving back and forth from vector
registers.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@297026
91177308-0d34-0410-b5e6-
96231b3b80d8
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