InstCombine: Stop two transforms dueling

InstCombineMulDivRem has:
// Canonicalize (X+C1)*CI -> X*CI+C1*CI.

InstCombineAddSub has:
// W*X + Y*Z --> W * (X+Z)  iff W == Y

These two transforms could fight with each other if C1*CI would not fold
away to something simpler than a ConstantExpr mul.

The InstCombineMulDivRem transform only acted on ConstantInts until
r199602 when it was changed to operate on all Constants in order to
let it fire on ConstantVectors.

To fix this, make this transform more careful by checking to see if we
actually folded away C1*CI.

This fixes PR20079.