introduce a common superclass, SILNode.
This is in preparation for allowing instructions to have multiple
results. It is also a somewhat more elegant representation for
instructions that have zero results. Instructions that are known
to have exactly one result inherit from a class, SingleValueInstruction,
that subclasses both ValueBase and SILInstruction. Some care must be
taken when working with SILNode pointers and testing for equality;
please see the comment on SILNode for more information.
A number of SIL passes needed to be updated in order to handle this
new distinction between SIL values and SIL instructions.
Note that the SIL parser is now stricter about not trying to assign
a result value from an instruction (like 'return' or 'strong_retain')
that does not produce any.
The problem here is that we were performing a naive negative FileCheck test for
retain/release. In certain modes, we would not have any retains/releases along
normal control paths but would have retains on unreachable paths. This test only
is trying to test if normal code paths have this issue.
To work around this issue, I created a small utility pass that prunes all
non-unreachable instructions from blocks with an unreachable terminator. This is
useful functionality in general when analyzing SIL since often times one will
have large fatal error blocks that disguise the true behavior of the
function. In this specific case, I just pipe in the normal sil output and run it
through sil-opt. sil-opt then runs just the utility pass and I then FileCheck
that sil-opt output.
rdar://30181104
