[SPIR-V] Legalize G_PHI of a vector of pointers (#241166)
With `SPV_INTEL_masked_gather_scatter`, vectors of pointers are legal
types and `G_IMPLICIT_DEF`/`G_FREEZE` already accept them, but `G_PHI`
did not, so a PHI of `<N x ptr>` failed with "unable to legalize
instruction". The SLP vectorizer forms such PHIs from vectorized address
computations. This adds the same pointer-vector rule to `G_PHI`.
Assisted-by: Claude Opus 5.5
