[llvm] [SPIR-V] Move structurizer to ISel prepare (PR #157886)

[Nathan Gauër via llvm-commits]

Keenuts wrote:

Rebased on main. There was one SPIR-V test failure I could not reproduce locally. Will see if this has been fixed, or if I need to dig into it first.

https://github.com/llvm/llvm-project/pull/157886