unwinds to in the CFG

Hi Nick,

Just as a quick recap the problem I encountered is how to deal
instructions in a block being used as operands in the unwind dest. Such
as this:

bb1: unwinds to %cleanup
call void @foo() ; might throw, might not
%x = add i32 %y, %z
call void @foo() ; might throw, might not
ret void
call void @use(i32 %x)

The problem is that %x might not have been executed before we enter

how about just declaring this illegal? i.e. require the first call
to be in a different basic block to the second, making it possible
to use a phi node in %cleanup.



I pretty much support your opinion Duncan. Unwind
BBs should not mess with values defined by their worker
BB. We should not introduce undefined behaviour intentionally.