rs6000: Change the length attribute default

rs6000: Change the length attribute default

This moves what is currently the default of the length attribute to
the only branch instruction patterns where it applies, namely, the
B-form instructions.  It was used for the "jump" instruction as well
before, but that is an I-form instruction and always has length 4.

	* config/rs6000/rs6000.md (length): Always define as const_int 4.
	(unnamed conditional branch define_insn): Set length to 4 or 8
	depending on offset.
	(<bd>_<mode>): Similar, for alternative 0.
	(<bd>tf_<mode>): Ditto.

From-SVN: r263601