[mlir][spirv] Add patterns to convert `spirv.*MulExtended` to `spirv.MulI`

[kuhar]

When targeting SPIR-V for WebGPU, we can't rely on WGSL's support for extended multiplication: gpuweb/gpuweb#1565.

To work around this limitation, we should provide patterns to rewrite spirv.*MulExtended in terms of regular multiplication. We need both the Signed and Unsigned variants, because the former is used in TosaToArith while the latter in wide integer emulation.

[llvmbot]

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