Generalize assignment decomposition to handle arbitrary combinations of
physically promoted structs. Do this by introducing a DecompositionPlan
class that keeps track of the copies to do that involve replacement
fields. The first step is then to fill out this plan. In the general
case where both the source and destination are physically promoted this
involves iterating the replacements in lockstep. For promotions that map
exactly, a direct copy between their locals is queued into the plan; in
other cases (e.g. partial overlap) it may involve writing the source
back to the struct local.

The plan is used to generate the IR and to figure out whether the writes
of all the replacement fields covers the destination such that the
original struct copy can be omitted.

Also unnest StatementList and rename it to DecompositionStatementList as
the other name conflicted with another class.

The JIT has a flag GTF_ICON_INITCLASS that represents that accesses off
that address are cctor dependent. Hoisting uses this to avoid hoisting
cctor dependent indirections unless all cctors are also hoisted.
However, local constant prop/VN-based constant prop do not handle this
flag, so we could run into cases where addresses with GTF_ICON_INITCLASS
were propagated and then subsequently hoisted incorrectly.

This change moves the flag to an OperIsIndir() flag instead of being a
flag on the constant. After some digging, I found that the original
reason the flag was not an indir flag was simply that there were no more
indir flags available, but we do have available flags today. This fix
is much simpler than the alternatives which would be to teach VN/local
copy prop to propagate this GTF_ICON_INITCLASS flag.

Also remove GTF_FLD_INITCLASS which is never set.

…composition

…composition

…composition-remainder

…composition

This reverts commit 2afbf82.

This reverts commit d252517.

…composition

…composition