x86: Simplify check for distinct TMM register operands

If any pair of operands in AMX instructions with 3 TMM register operands
are the same, the instruction will UD.  Don't call register_number to
check for distinct TMM register operands since all TMM register operands
have the same size.

	* config/tc-i386.c (check_VecOperands): Remove register_number
	call when checking for distinct TMM register operands.