Returns cost of address in shape symbol + var + OFFSET + RATIO * index.
If SYMBOL_PRESENT is false, symbol is omitted. If VAR_PRESENT is false,
variable is omitted. Compute the cost for a memory reference that accesses
a memory location of mode MEM_MODE in address space AS.
MAY_AUTOINC is set to true if the autoincrement (increasing index by
size of MEM_MODE / RATIO) is available. To make this determination, we
look at the size of the increment to be made, which is given in CSTEP.
CSTEP may be zero if the step is unknown.
STMT_AFTER_INC is true iff the statement we're looking at is after the
increment of the original biv.
TODO -- there must be some better way. This all is quite crude.