This patch introduces the field `ExpressionSize` in SCEV. This field is calculated only once on SCEV creation,

and it represents the complexity of this SCEV from arithmetical point of view (not from the point of the number

of actual different SCEV nodes that are used in the expression). Roughly saying, it is the number of operands

and operations symbols when we print this SCEV.

A formal definition is following: if SCEV `X` has operands `Op1`, `Op2`, ..., `OpN`, then

Size(X) = 1 + Size(Op1) + Size(Op2) + ... + Size(OpN)

here `N` can also be zero if `X` is a constant or SCEVUnknown.

Expression size may be used as a universal way to limit SCEV transformations for huge SCEVs. Currently,

we have a bunch of options that represents various limits (such as recursion depth limit) that may not

make any sense from the point of view of a LLVM users who is not familiar with SCEV internals, and all

these different options pursue one goal. A more general rule that may potentially allow us to get rid of

this redundancy in options is "do not make transformations with SCEVs of huge size". It can apply to all

SCEV traversals and transformations that may need to visit a SCEV node more than once, hence they are

prone to combinatorial explosions.

This patch only introduces SCEV sizes calculation as NFC, its utilization will be introduced in follow-up patches.