rs6000: Remove "length 4" from other insns

rs6000: Remove "length 4" from other insns

There were many insns that set "length 4" explicitly while that does
not make anything clearer to the reader.  So, simplify the code.

	* config/rs6000/altivec.md: Don't set length attribute to the default
	value.
	* config/rs6000/darwin.md: Ditto.
	* config/rs6000/dfp.md: Ditto.
	* config/rs6000/htm.md: Ditto.
	* config/rs6000/rs6000.md: Ditto.
	* config/rs6000/sync.md: Ditto.
	* config/rs6000/vsx.md: Ditto.

From-SVN: r263603