[AArch64] Fix bogus MOVPRFX warning for GPR form of CPY

[AArch64] Fix bogus MOVPRFX warning for GPR form of CPY

One of the MOVPRFX tests has:

  output register of preceding `movprfx' used as input at operand 3 -- `cpy z1.d,p1/m,x1'

But X1 and Z1 are not the same register, so the instruction is
actually OK.

2019-07-02  Richard Sandiford  <richard.sandiford@arm.com>

opcodes/
	* aarch64-opc.c (verify_constraints): Skip GPRs when scanning the
	registers in an instruction prefixed by MOVPRFX.

gas/
	* testsuite/gas/aarch64/sve-movprfx_25.s: Allow CPY Z1.D.P1/M,X1
	to be prefixed by MOVPRFX.
	* testsuite/gas/aarch64/sve-movprfx_25.d: Update accordingly.
	* testsuite/gas/aarch64/sve-movprfx_25.l: Likewise.