…ity function

Replace the `isa(SILNode *)` with `isa(SILInstruction *)` and `isa(SILValue)`.
This is much clearer and it also works if the SILValue is a MultiValueInstructionResult of an apply instruction.

Also, use `isa` instead of `classof` in canOptimize()

Split `markValueLive(SILNode *)` into `markValueLive(SILValue)` and `markInstructionLive(SILInstruction*)`

… when checking for invariant arguments.

Use `getDefiningInstruction` instead of casting a a SILValue to SingleValueInstruction.

…om SILNode* to SILValue

A small cleanup, NFC.

NFC

This resolves a FIXME.
Also, use `getDefiningInstruction()` instead of `getRepresentativeSILNodeInObject()`

NFC

…tUsersOfInstruction.

A small cleanup, NFC.

…tion *)`

A small cleanup, NFC

A small cleanup, NFC.

This removes the ambiguity when casting from a SingleValueInstruction to SILNode, which makes the code simpler. E.g. the "isRepresentativeSILNode" logic is not needed anymore.
Also, it reduces the size of the most used instruction class - SingleValueInstruction - by one pointer.

Conceptually, SILInstruction is still a SILNode. But implementation-wise SILNode is not a base class of SILInstruction anymore.
Only the two sub-classes of SILInstruction - SingleValueInstruction and NonSingleValueInstruction - inherit from SILNode. SingleValueInstruction's SILNode is embedded into a ValueBase and its relative offset in the class is the same as in NonSingleValueInstruction (see SILNodeOffsetChecker).
This makes it possible to cast from a SILInstruction to a SILNode without knowing which SILInstruction sub-class it is.
Casting to SILNode cannot be done implicitly, but only with an LLVM `cast` or with SILInstruction::asSILNode(). But this is a rare case anyway.