This change moves the simple constant analysis that was used by the const_func test into a pyrameterized module for use on any stage of the IR. This will be used to detect unreachable code.
@rdmarsh2 has been working on various queries and libraries on top of the IR, and has pointed out that having to always refer to an operand of an instruction by the pair of (instruction, operandTag) makes using the IR a bit clunky. This PR adds a new `Operand` IPA type that represents an operand of an instruction. `OperandTag` still exists, but is now an internal type used only in the IR implementation.
This change makes the public IR.qll module resolve to the flavor of the IR that we want queries to use. Today, this is the aliased SSA flavor of the IR. Should we add additional IR iterations in the future, we'll update IR.qll to resolve to whichever one we consider the default.
I moved the PrintIR.ql and IRSanity.ql queries into the internal directories of the corresponding flavors. There's still a PrintIR.ql and an IRSanity.ql in the public IR directory, which use the same IR flavor as the public IR.qll.
There are no real code changes here, other than to fix up `import`s. All tests still hae the same output, as expected.
A future commit will hide the IR flavors other than the one we want queries to use directly.
