Adds a virtual method, GetFreshIds(), to Transformation. Every
transformation uses this to indicate which ids in its protobuf message
are fresh ids. This means that when replaying a sequence of
transformations the replayer can obtain a smallest id that is not in
use by the module already and that will not be used by any
transformation by necessity. Ids greater than or equal to this id
can be used as overflow ids.
Fixes#3851.
Support identical predecessors in TransformationPropagateInstructionUp.
A basic block may have multiple identical predecessors as follows:
%1 = OpLabel
OpSelectionMerge %2 None
OpBranchConditional %true %2 %2
%2 = OpLabel
...
This case wasn't supported before.
Given an instruction (that may use an OpPhi result from the same block as an input operand), try to clone the instruction into each predecessor block, replacing the input operand with the corresponding OpPhi input operand in each case, if necessary.
Fixes#3458.
