Back when Vulkan was developed, there were a bunch of OpenGL drivers out there which had random AST parsing bugs (Dolphin even has a bunch of workarounds for them); So a large chunk of the motivation for SPIR-V was avoiding the need for every driver to implement their own GLSL parser and the associated bugs.
The problem for Dolphin is not the complexity of the shader, but the quantity.
Shaders in modern games are usually written manually (or authored in a shader node editor by an artist), so it's rare for a game to have more than a few thousand total. Better games might only have a few dozen for the entire game.
But because gamecube/Wii games configure the TEV pixel pipeline though a dynamic API, some games use that API in a pattern where Dolphin can find itself generating hundreds of shaders per second. Some games even manage to generate new shaders continually as you play, because they append junk state to their pixel pixeline state which dolphin doesn't detect as a duplicate.
