[i386] Check RegVRex in register_number

Increment register number by 16 if RegVRex is set.

Backport from master

	* config/tc-i386.c (register_number): Check RegVRex.
	* testsuite/gas/i386/x86-64-avx512f.s: Add a test for vgatherqpd
	with %zmm19 and %zmm3.
	* testsuite/gas/i386/x86-64-avx512f-intel.d: Updated.
	* testsuite/gas/i386/x86-64-avx512f.d: Likewise.