[ARM] Allow immediate without prefix in unified syntax for VCMP

2017-02-13  Thomas Preud'homme  <thomas.preudhomme@arm.com>

	gas/
	* config/tc-arm.c (parse_ifimm_zero): Make prefix optional in unified
	syntax.
	* testsuite/gas/arm/vcmp-noprefix-imm.d: New file.
	* testsuite/gas/arm/vcmp-noprefix-imm.s: New file.