[assert-instr] compare only the instruction prefix
[assert-instr] compare only the instruction prefix

When comparing the assembly instructions against
the expected instruction, depending on the platform,
we might end up with `tzcntl != tzcnt`. This
commit truncates the instructions to the length
of the expected instruction, such that `tzcntl => tzcnt`
and the comparison succeeds.

rust-lang/stdarch,rust-lang/stdarch,rust-lang/stdarch,rust-lang/stdarch,rust-lang/stdarch