i386: Add non-optimize prefetchi intrins

Under -O0, with the "newly" introduced intrins, the variable will be
transformed as mem instead of the origin symbol_ref. The compiler will
then treat the operand as invalid and turn the operation into nop, which
is not expected. Use macro for non-optimize to keep the variable as
symbol_ref just as how prefetch intrin does.

gcc/ChangeLog:

	* config/i386/prfchiintrin.h
	(_m_prefetchit0): Add macro for non-optimized option.
	(_m_prefetchit1): Ditto.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/prefetchi-1b.c: New test.