rs6000: Improve scc isel

rs6000: Improve scc isel

If we have a negative condition we can use a literal 0 in the isel,
instead of having to load it into a register.  If the condition is from
a comparison with an immediate we can change e.g. LT to LE and adjust
the immediate, saving a li instruction.

	* config/rs6000/rs6000.md (<code><GPR:mode><GPR2:mode>2_isel): Change
	LT/GT/LTU/GTU to LE/GE/LEU/GEU where possible.

From-SVN: r255186