Issue #667
Wires in a basic framework for destination-passing style, with
backwards-compatibility to the old approach, so that expression types
can be moved over to it one at a time (by moving them from trans_expr
to trans_expr_dps).
The builder functions in trans_build now look at an 'unreachable' flag
in the block context and don't generate code (returning undefined
placeholder values) when this flag is set. Threading the unreachable
flag through context still requires some care, but this seems a more
sane approach than re-checking for terminated blocks throughout the
compiler.
When creating a block, if you use its closest dominator as parent, the
flag will be automatically passed through. If you can't do that,
because the dominator is a scope block that you're trying to get out
of, you'll have to do something like this to explicitly pass on the
flag:
if bcx.unreachable { Unreachable(next_cx); }
Closes#949. Closes#946. Closes#942. Closes#895. Closes#894.
Closes#892. Closes#957. Closes#958.
You now do
bld::Ret(bcx, someval)
where you used to say
bcx.build.Ret(someval)
Two fewer boxes are allocated for each block context, and build calls
no longer go through a vtable.
