Add check for 8-bit old registers in EVEX format

Since APX supports EVEX from legacy instructions, we need to check
the 8-bit old registers in EVEX format. And add test cases for it.

gas/ChangeLog:

        * config/tc-i386.c (md_assemble): Add invalid check for old byte
        registers in EVEX format.
        * testsuite/gas/i386/x86-64-apx-inval.l: Add new test.
        * testsuite/gas/i386/x86-64-apx-inval.s: Ditto.