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.
With the option -sil-print-debuginfo the printing of debug locations and scopes can be enabled.
I made the default for the option “false”, because in 99% of the time I don’t need the debug info in the printed SIL and I prefer better readability.
Introduce a new SILPrintContext class which is the main handle passed to the SILModule's and SILFunction's print functions.
It also allows to let derived classes implement call backs on instruction printing.
NFC for now, but needed for the upcoming SIL-debuginfo change.
