[AArch64][SVE] Teach copyPhysReg to copy ZPR2/3/4.

It's sort of tricky to hit this in practice, but not impossible. I have
a synthetic C testcase if anyone is interested.

The implementation is identical to the equivalent NEON register copies.

Differential Revision: https://reviews.llvm.org/D84373

(cherry picked from commit 993c1a32)