testsuite: Fix sve/mul_2.c failures

testsuite: Fix sve/mul_2.c failures

Some sve/mul_2.c tests were failing because we'd (reasonably)
decided to use shifts and adds instead of MULs for some simple
negative constants.  We'd already needed to avoid that when
picking positive constants, so this patch does the same thing
for the negative ones.

gcc/testsuite/
	* gcc.target/aarch64/sve/mul_2.c: Adjust negative constants to avoid
	conversion to shifts and adds.