[MLIR][SPIRV] Rename `spv.undef` to `spv.Undef`.

To unify the naming scheme across all ops in the SPIR-V dialect, we are
moving from spv.camelCase to spv.CamelCase everywhere. For ops that
don't have a SPIR-V spec counterpart, we use spv.mlir.snake_case.

Reviewed By: antiagainst

Differential Revision: https://reviews.llvm.org/D98016