x86: Optimize with EVEX128 encoding for AVX512VL

x86: Optimize with EVEX128 encoding for AVX512VL

We can optimize AVX512 instructions with EVEX128 only if AVX512VL is
enabled:

1. Instruction is an AVX512VL instruction. Or
2. AVX512VL is enabled explicitly by -march=+avx512vl/".arch .avx512vl".

We should optimize EVEX instructions with EVEX128 encoding when pseudo
{evex} prefix is used.

	* config/tc-i386.c (set_cpu_arch): Set cpu_arch_isa_flags.
	(md_parse_option): Likewise.
	(optimize_encoding): Check i.tm.cpu_flags and cpu_arch_isa_flags
	for cpuavx512vl instead of cpu_arch_flags.  Optimize EVEX with
	EVEX128 when EVEX encoding is required.
	* testsuite/gas/i386/i386.exp: Run optimize-4, optimize-5,
	x86-64-optimize-5 and x86-64-optimize-6.
	* testsuite/gas/i386/optimize-1.d: Updated.
	* testsuite/gas/i386/x86-64-optimize-2.d: Likewise.
	* testsuite/gas/i386/optimize-4.d: New file.
	* testsuite/gas/i386/optimize-4.s: Likewise.
	* testsuite/gas/i386/optimize-5.d: Likewise.
	* testsuite/gas/i386/optimize-5.s: Likewise.
	* testsuite/gas/i386/x86-64-optimize-5.d: Likewise.
	* testsuite/gas/i386/x86-64-optimize-5.s: Likewise.
	* testsuite/gas/i386/x86-64-optimize-6.d: Likewise.
	* testsuite/gas/i386/x86-64-optimize-6.s: Likewise.