rs6000: Improve *lt0 patterns

rs6000: Improve *lt0 patterns

The rs6000 port currently has an *lt0_disi define_insn, setting the DI
result to whether the SI argument is negative or not.  It turns out the
generic optimisers cannot always figure out in the other cases either
that this is just a shift for us.  This patch adds patterns for all
four SI/DI combinations.

	* config/rs6000/rs6000.md (*lt0_disi): Delete.
	(*lt0_<mode>di, *lt0_<mode>si): New.

From-SVN: r254374