Errored: OpenSmalltalk/opensmalltalk-vm#1974 (Cog - b855702)

CogVM source as per VMMaker.oscog-eem.2731

Cogit:
Refactor the compilation breakpoints so they don't take a selector length.
Leave it up to CoInterpeeter to derive the byte length of the selector using
numBytesOf:. Hence no longer mark lengthOf: as <api>.

Use a writable method when setting cpicHasMNUCase: in
cogExtendPIC:CaseNMethod:tag:isMNUCase:. ARMv8 is getting there...

Slang: do constant folding in left and right shifts.

Interpreters: rip out the check alloc filler support in the interests of
simplicity. Though useful in theory, this faclity has never been used
successfully to find a bug in all the years it's been available (and we
can always put it back).
Rip out the debugPrimCallStackOffset support. I can't remember what this
was supposed to do. It is confusing.

Spur: Don't update become effect flags for identical oops that will later
be filtered out in the loops oiver the arrays. If newFinalizartion is set,
then don't queue a WeakArray for finalization more than once (i.e. if it's
already in the queue there's no point adding it again).

Cogit: fix genExternalizePointersForPrimitiveCall &
genLoadCStackPointersForPrimCall for the SPReg ~= NativeSPReg regime.
Localise code to the primitive generator invocation block in compilePrimitive.