Fix instruction length for MMA insns.

Prefixed instructions should not have their length explicitly set to '8'. The function get_attr_length() will adjust the length appropriately based on the value of the "prefixed" attribute.

2020-12-16  Pat Haugen  <pthaugen@linux.ibm.com>

gcc/
	* config/rs6000/mma.md (*movxo, mma_<vvi4i4i8>, mma_<avvi4i4i8>,
	mma_<vvi4i4i2>, mma_<avvi4i4i2>, mma_<vvi4i4>, mma_<avvi4i4>,
	mma_<pvi4i2>, mma_<apvi4i2>, mma_<vvi4i4i4>, mma_<avvi4i4i4>):
	Remove explicit setting of length attribute.