rs6000/test: Add -fno-vect-cost-model to conv-vectorize-[12].c

rs6000/test: Add -fno-vect-cost-model to conv-vectorize-[12].c

In my testing with cost tweaking for vector with length, I found
two cases below didn't get the expected output.  Since the expected
instructions reply on the vectorization occurrence, we don't expect
vectorization gets disabled by cost model.

To make it not fragile, the fix is to force it without vect cost model.

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/conv-vectorize-1.c: Add option
	-fno-vect-cost-model.
	* gcc.target/powerpc/conv-vectorize-2.c: Likewise.