[DAG] Restrict (fp_round (copysign X, Y)) -> (copysign (fp_round X), Y) combine

This transformation creates an copysign node whose argument types do not match. RISCV does not handle such a case which results in a crash today. Looking at the relevant code in DAG, it looks like the process of enabling the non-matching types case was never completed for vectors at all. The transformation which triggered the RISCV crash is a specialization of another transform (specifically due to one use for profitability) which isn't enabled by default. Given that, I chose to match the preconditions for that other transform.

Other options here include:
* Updating RISCV codegen to handle the mismatched argument type case for vectors. This is slightly tricky as I don't see an obvious profitable lowering for this case which doesn't involve simply adding back in the round/trunc.
* Disabling the transform via a target hook.

This patch does involve two changes for AArch64 codegen. These could be called regressions, but well, the code after actually looks better than the code before.

Differential Revision: https://reviews.llvm.org/D148638

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