X86: Ignore REX_B bit for 32-bit XOP instructions

While decoding 32-bit XOP instructions, 64 bit registers names are printed.
This patch fixes this by ignoring REX_B bit in 32-bit mode.

opcodes/

	PR binutils/20637
	* i386-dis.c (get_valid_dis386): Ignore REX_B for 32-bit XOP
	instructions.

gas/

	PR binutils/20637
	* testsuite/gas/i386/xop32reg.d: New file.
	* testsuite/gas/i386/xop32reg.s: New file.
	* testsuite/gas/i386/i386.exp: Run new test.