nir/instr_set: remove unnecessary check in nir_instrs_equal()

nir/instr_set: remove unnecessary check in nir_instrs_equal()

This was originally added to nir_instrs_equal() instead of
nir_instr_can_cse() incorrectly, but this was fixed when moving to the
instruction set API (as it had to be, otherwise hashing wouldn't work).
Now, this is dead code since instr_can_rewrite() will only return true
for texture instructions that use an index, so we can turn the check into
an assert. This also means that now nir_instrs_equal(instr, instr) will
always return true unless it assert-fails.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Signed-off-by: Connor Abbott <cwabbott0@gmail.com>