I did forget to mention the requirement that you can't branch into the middle of an instruction. If you have both of these constraints then you can unambiguously determine the location of all instructions in any aligned 64-byte block, including at I$ fill time.

Implementing this would require instruction fetch to take an exception on line-crossing instructions (which must be illegal) and a change to the assembler to insert a 16-bit padding nop or expand a compressed instruction to maintain the alignment. There is nothing needed from the compiler (or linker AFAICT). JITs will have to be aware though.