Duffcopy now uses X0, as of 5cf281a9.  Teach the peephole
optimizer that duffcopy clobbers X0 so that it does not
rename registers use X0 across the duffcopy instruction.

Fixes #13171

Change-Id: I389cbf1982cb6eb2f51e6152ac96736a8589f085
Reviewed-on: https://go-review.googlesource.com/16715
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Ilya Tocar <ilya.tocar@intel.com>