SSARewriter needs to handle id overflows

Doesn't seem like SSARewriter handles id overflows, which results in it creating invalid spirv, which then crashes further passes. The TODO in SSARewriter::CreatePhiCandidate mentions issue #1841, but that's been merged already, so I'm opening this as a separate issue for that specific pass.

Start by locating SSARewriter::CreatePhiCandidate and read the TODO that references issue #1841. Trace how IDs are created and how overflow is handled in this pass. Done means overflow no longer produces invalid SPIR-V that crashes later passes, with the relevant pass behavior covered by tests if existing test locations are found.

Written by the indexing model from the issue text.