rs6000: Define define_insn_and_split to split unspec sldi+or to rldimi

rs6000: Define define_insn_and_split to split unspec sldi+or to rldimi

Combine pass could recognize the pattern defined and split it in split1,
this patch could optimize:

21: r130:DI=r133:DI<<0x20
11: {r129:DI=zero_extend(unspec[[r145:DI]] 87);clobber scratch;}
22: r134:DI=r130:DI|r129:DI

to

21: {r149:DI=zero_extend(unspec[[r145:DI]] 87);clobber scratch;}
22: r134:DI=r149:DI&0xffffffff|r133:DI<<0x20

rldimi is generated instead of sldi+or.

gcc/ChangeLog:

2020-07-13  Xionghu Luo  <luoxhu@linux.ibm.com>

	* config/rs6000/rs6000.md (rotl_unspec): New
	define_insn_and_split.

gcc/testsuite/ChangeLog:

2020-07-13  Xionghu Luo  <luoxhu@linux.ibm.com>

	* gcc.target/powerpc/vector_float.c: New test.