amdgcn: Add instruction pattern for conditional shift operations

amdgcn: Add instruction pattern for conditional shift operations

gcc/ChangeLog:

	* config/gcn/gcn-valu.md (cond_<expander><mode>): Add
	cond_{ashl|ashr|lshr}

gcc/testsuite/ChangeLog:

	* gcc.target/gcn/cond_shift_3.c: New test.
	* gcc.target/gcn/cond_shift_3_run.c: New test.
	* gcc.target/gcn/cond_shift_4.c: New test.
	* gcc.target/gcn/cond_shift_4_run.c: New test.
	* gcc.target/gcn/cond_shift_8.c: New test.
	* gcc.target/gcn/cond_shift_8_run.c: New test.
	* gcc.target/gcn/cond_shift_9.c: New test.
	* gcc.target/gcn/cond_shift_9_run.c: New test.