pru: Optimize DI shifts

pru: Optimize DI shifts

If the number of shift positions is a constant, then the DI shift
operation is expanded to a sequence of 2 to 4 machine instructions.
That is more efficient than the default action to call libgcc.

gcc/ChangeLog:

	* config/pru/pru.md (lshrdi3): New expand pattern.
	(ashldi3): Ditto.

gcc/testsuite/ChangeLog:

	* gcc.target/pru/ashiftdi-1.c: New test.
	* gcc.target/pru/lshiftrtdi-1.c: New test.

Signed-off-by: Dimitar Dimitrov <dimitar@dinux.eu>