testsuite: Compile slsr-39.c without vectorisation

testsuite: Compile slsr-39.c without vectorisation

The fix for PR106019 regressed slsr-39.c for -m32 -march=cascadelake
because we are now able to vectorise the code.  (Whether the code model
should be allowing that is a different question -- the vectorised code
looked worse to me.)

The test runs at -O2 and predates vectorisation being enabled at -O2,
so this patch just adds -fno-tree-vectorize.

gcc/testsuite/
	* gcc.dg/tree-ssa/slsr-39.c: Force vectorization off.