[DAGCombiner] Don't fold FCOPYSIGN vector sign operand casts
Avoid doing the following combine for vector types:
copysign(x, fp_extend(y)) -> copysign(x, y) copysign(x, fp_round(y)) -> copysign(x, y)
That combine seemed to impede the selection of vector instruction and cause
a mess in some circumstances.
Differential Revision: https://reviews.llvm.org/D96037