Validator: allow unreachable continue construct under certain conditions

[dneto0]

This issue tracks validator changes for changes expected from resolution to KhronosGroup/SPIRV-Headers#16

Related to
#270

Currently, the validator disallows an unreachable continue construct for a loop. I expect we should relax this rule.

[dneto0]

With b51b809 the validator now will find an unreachable continue construct.

It finds candidate back-edges via a special forward depth-first-search in the augmented CFG that also adds an edge from any block with an OpLoopMerge %merge %cont instruction to the named continue target %cont. But that DFS is not used for computing dominance.

[dneto0]

Still need to take this feedback to the SPIR working group to resolve KhronosGroup/SPIRV-Headers#16