[SPIRV] Use a worklist in the post-legalizer

This commit refactors the SPIRV post-legalizer to use a worklist to process
new instructions. Previously, the post-legalizer would iterate through all
instructions and try to assign types. This could fail if a new instruction
depended on another new instruction that had not been processed yet.

The new implementation adds all new instructions that require a SPIR-V type
to a worklist. It then iteratively processes the worklist until it is empty.
This ensures that all dependencies are met before an instruction is
processed.

This change makes the post-legalizer more robust and fixes potential ordering
issues with newly generated instructions.

Existing tests cover existing functionality. More tests will be added as
the legalizer is modified.

Part of #153091

Author: s-perron