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Previously SandboxBPF constructed the CodeGen Instruction sequence
primarily in one large, complex function.  Additionally, it made
extensive use of the CodeGen::JoinInstructions() function to
conditionally arrange various bits of instructions, which made it
harder to follow.

This CL splits the Instruction assembly code into 5 mostly distinct
functions and eliminates all use of JoinInstruction() in favor of
function composition.  E.g., instead of

    foo = gen->MakeInstruction(...);
    bar = gen->MakeInstruction(...);
    gen->JoinInstructions(foo, bar);

this CL favors writing

    MakeFoo(MakeBar())

with the convention that Instruction-constructing functions should
arrange for control to transfer to the Instruction sequence argument
when complete.  (I.e., "continuation-passing style":
http://en.wikipedia.org/wiki/Continuation-passing_style)

BUG=414363

Review URL: https://codereview.chromium.org/568053005

Cr-Commit-Position: refs/heads/master@{#295015}