Sure, but I think the instruction length specification would still be *very*
useful in some cases to lock things down (and document the length
dependency).
Instruction length can a vague constraint:
0: c4 e1 78 28 50 00 vmovaps 0x0(%rax),%xmm2
6: 62 f1 7c 08 28 10 vmovaps (%rax),%xmm2
Both instructions with the same length do the same thing.
But one is AVX and the other is AVX512.
c: c4 e1 78 28 10 vmovaps (%rax),%xmm2
11: c5 f8 28 50 00 vmovaps 0x0(%rax),%xmm2
These 2 are AVX. Majority of programmers don't care how
an instruction is encoded. If they want specific instruction
length, they can experiment with pseudo prefixes to get
exactly what they need.