avoid infinite looping when folding vector multiplies of constants (PR22698)

avoid infinite looping when folding vector multiplies of constants (PR22698)

We were missing a check for the following fold in DAGCombiner:

// fold (fmul (fmul x, c1), c2) -> (fmul x, (fmul c1, c2))

If 'x' is also a constant, then we shouldn't do anything. Otherwise, we could end up swapping the operands back and forth forever.

This should fix:
http://llvm.org/bugs/show_bug.cgi?id=22698

Differential Revision: http://reviews.llvm.org/D7917

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230884 91177308-0d34-0410-b5e6-96231b3b80d8