[mlir][spirv] Support poison index when converting vector.insert/extract (#125560)

This modifies the conversion patterns so that, in the case where the
index is known statically to be poison, the insertion/extraction is
replaced by an arbitrary junk constant value, and in the dynamic case,
the index is sanitized at runtime. This avoids triggering a UB in both
cases. The dynamic case is definitely a pessimisation of the generated
code, but the use of dynamic indexes is expected to be very rare and
already slow on real-world GPU compilers ingesting SPIR-V, so the impact
should be negligible.

Resolves #124162.

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