Document in the context what happens with id overflow. (#2159)

Added documentation to the ir context to indicates that TakeNextId()
returns 0 when the max id is reached.  TODOs were added to each call
sight so that we know where we have to start to handle this case.

Handle id overflow in |SplitLoopHeader|.

Handle id overflow in |GetOrCreatePreHeaderBlock|.

Handle failure to create preheader in LICM.

Part of https://github.com/KhronosGroup/SPIRV-Tools/issues/1841.