We have the following DAGCombiner transformations:
(mul (shl X, c1), c2) -> (mul X, c2 << c1) (mul (shl X, C), Y) -> (shl (mul X, Y), C) (shl (mul x, c1), c2) -> (mul x, c1 << c2)
Usually the constant shift is optimised by SelectionDAG::getNode when it is constructed, by SelectionDAG::FoldConstantArithmetic, but when we're dealing with vectors and one of those vector constants contains an undef element FoldConstantArithmetic does not fold and we enter an infinite loop.
Fix this by making FoldConstantArithmetic use getNode to decide how to fold each vector element, the same as FoldConstantVectorArithmetic does, and rather than adding the constant shift to the work list instead only apply the transformation if it's already been folded into a constant, as if it's not we're going to loop endlessly. Additionally add missing NoOpaques to one of those transformations, which I noticed when writing the tests for this.