x86: Correct EVEX to 128-bit EVEX optimization

x86: Correct EVEX to 128-bit EVEX optimization

Since not all AVX512F processors support AVX512VL, we can optimize
512-bit EVEX to 128-bit EVEX encoding for upper 16 vector registers
only when AVX512VL is enabled explicitly at command-line or via
".arch .avx512vl" directive.

	PR gas/24352
	* config/tc-i386.c (optimize_encoding): Check only
	cpu_arch_flags.bitfield.cpuavx512vl.
	* testsuite/gas/i386/i386.exp: Run x86-64-optimize-2b.
	* testsuite/gas/i386/x86-64-optimize-2.d: Revert the last
	change.
	* testsuite/gas/i386/x86-64-optimize-2b.d: New file.
	* testsuite/gas/i386/x86-64-optimize-2b.s: Likewise.