rs6000: Clean up the pre-checkings of expand_block_compare

Remove P7 CPU test as only P7 above can enter this function and P7 LE is
excluded by the checking of targetm.slow_unaligned_access on word_mode.
Also performance test shows the expand of block compare is better than
library on P7 BE when the length is from 16 bytes to 64 bytes.

gcc/
	* config/rs6000/rs6000-string.cc (expand_block_compare): Assert
	only P7 above can enter this function.  Remove P7 CPU test and let
	P7 BE do the expand.

gcc/testsuite/
	* gcc.target/powerpc/block-cmp-4.c: New.