This change fixes a few key problems with the existing SSA implementations:
For unaliased SSA, we were incorrectly choosing to model a local variable that had accesses that did not cover the entire variable. This has been changed to ensure that all accesses to the variable are at offset zero and have the same type as the variable itself. This was only possible to fix now that every `MemoryOperand` has its own type.
For aliased SSA, we now correctly track the offset and size of each memory access using an interval of bit offsets covered by the access. The offset interval makes the overlap computation more straightforward. Again, this is only possible now that operands have types.
The `getXXXMemoryAccess` predicates are now driven by the `MemoryAccessKind` on the operands and results, instead of by specific opcodes.
This change does fix an existing false negative in the IR dataflow tests.
I added a few simple test cases to the SSA IR tests, covering the various kinds of overlap (MustExcactly, MustTotally, and MayPartially).
I added "PrintSSA.qll", which can dump the SSA memory accesses as part of an IR dump.
The IR tests were getting kind of unwieldy. We were using "ir.cpp" to contain test cases that covered both IR construction (every language construct imaginable) and SSA construction. We would then build and dump all three flavors of IR. For IR construction tests, examining the SSA dumps when you add a new test case is tedious.
To make this easier to manage, I've split the SSA-specific test cases out into a separate directory. "ir.cpp" should now contain only IR construction test cases, and "ssa.cpp" should contain only SSA construction test cases. We dump just the raw IR for "ir.cpp", and just the two SSA flavors for "ssa.cpp". We still run all three flavors of the IR sanity tests for "ir.cpp", though.
I also removed the "ssa_block_count.ql" test, which wasn't really adding any coverage, because any change to the block count would be reflected in the dump as well.
