Diagnose when trying to assemble conditional FP16 vmovx and vins

This patch makes GAS emit a warning when trying to assemble the Armv8.2
FP16 instructions VMOVX and VINS with condition codes. The Armv8-A
Reference Manual specifies these instructions without conditional codes
and says that if they are found in an IT block that they are CONSTRAINED
UNPREDICABLE.

gas/ChangeLog:
2018-02-22  Andre Vieira  <andre.simoesdiasvieira@arm.com>

	* config/tc-arm.c (do_neon_movhf): If conditional error out when in arm
	mode and emit warning in thumb mode.
	* testsuite/gas/arm/armv8-2-fp16-scalar-bad.s: Add new tests.
	* testsuite/gas/arm/armv8-2-fp16-scalar-bad.l: Idem.