Ensure the dominance calculation visits all nodes in the CFG.
The successor list of the pseudo-entry node is augmented with
a single node in each cycle that otherwise would not be visited.
Similarly, the predecssors list of the pseduo-exit node is augmented
with the a single node in each cycle that otherwise would not
be visited.
Pulls DepthFirstSearch out so it's accessible outside of the dominator
calculation.
Fixes https://github.com/KhronosGroup/SPIRV-Tools/issues/279
* Creates an ID class which manages definition and use of IDs
* Moved tracking code from validate.cpp to validate_id.cpp
* Rename and combine SsaPass and ProcessIds into IdPass
* Remove module dependency in Function
For dominance calculations we use an "augmented" CFG
where we always add a pseudo-entry node that is the predecessor
in the augmented CFG to any nodes that have no predecessors in the
regular CFG. Similarly, we add a pseudo-exit node that is the
predecessor in the augmented CFG that is a successor to any
node that has no successors in the regular CFG.
Pseudo entry and exit blocks live in the Function object.
Fixes a subtle problem where we were implicitly creating
the block_details for the pseudo-exit node since it didn't
appear in the idoms map, and yet we referenced it. In such a case the
contents of the block details could be garbage, or zero-initialized.
That sometimes caused incorrect calculation of immediate dominators
and post-dominators. For example, on a debug build where the details
could be zero-initialized, the dominator of an unreachable block would
be given as the pseudo-exit node. Bizarre.
Also, enforce the rule that you must have an OpFunctionEnd to close off
the last function.
Refactor the way the post order vector is created. This new method
will allow for the extraction of backedges and create the post order
vector in one pass.
