rs6000: Generate rl*imi for memory some more

rs6000: Generate rl*imi for memory some more

An rl<wd>imi instruction is often written like "(a << 8) | (b & 255)".
If "b" now is a byte in memory, combine will combine the load with the
masking (with 255 in the example), since that is a single instruction;
and then the rl*imi isn't combined from the remaining pieces.

This patch adds a splitter to make combine handle this case.

	* config/rs6000/rs6000.md (splitters for rldimi and rlwimi with the
	zero_extend argument from memory): New.

From-SVN: r262929