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x86: Check for BMI1 and BMI2 flags in addition to AVX2

All known AVX2-capable CPU:s has BMI1 and BMI2, but apparently some
x86 emulators can be configured to emulate esoteric combinations of
instruction sets that doesn't correspond to any existing hardware.

--- a/src/x86/cpu.c

+++ b/src/x86/cpu.c

@@ -57,7 +57,8 @@

                 if (info[2] & (1 << 28)) flags |= DAV1D_X86_CPU_FLAG_AVX;

                 if (n_ids >= 7) {

                     dav1d_cpu_cpuid(info, 7);

-                    if (info[1] & (1 <<  5)) flags |= DAV1D_X86_CPU_FLAG_AVX2;

+                    if ((info[1] & 0x00000128) == 0x00000128)

+                        flags |= DAV1D_X86_CPU_FLAG_AVX2;

                     if ((xcr & 0x000000e0) == 0x000000e0) /* ZMM/OPMASK */ {

                         if ((info[1] & 0xd0030000) == 0xd0030000)

                             flags |= DAV1D_X86_CPU_FLAG_AVX512;