[LV] Make getSymbolicStrides return a pointer rather than a reference. NFC

Turns out SymbolicStrides is actually used in canVectorizeWithIfConvert
before it gets set up in canVectorizeMemory.

This works fine as long as SymbolicStrides resides in LV since we just
have an empty map.  Based on this the conclusion is made that there are
no symbolic strides which is conservatively correct.

However once SymbolicStrides becomes part of LAI, LAI is nullptr at this
point so we need to differentiate the uninitialized state by returning a
nullptr for SymbolicStrides.

llvm-svn: 272966