The previous shape of SequenceExpression only permitted expressions
in the sequence. This is pretty common in most ILs, however, it usually
leads to complicated manual spilling in the event that a statement is needed.
This is often necessary when, for example, a compiler is deeply nested in some
expression production, and then realizes the code expansion requires a
statement (e.g., maybe a new local variable must be declared, etc).
Instead of requiring complicated code-gen, this change permits SequenceExpression
to contain an arbitrary mixture of expression/statement prelude nodes, terminating
with a single, final Expression which yields the actual expression value. The
runtime bears the burden of implementing this which, frankly, is pretty trivial.
fde88b7cf4