nir: Actually do load/store vectorization beyond vec2

nir_opt_load_store_vectorize has an is_strided_vector() function that
looks for types with weird explicit strides.  It does so by comparing
the explicit stride against the type-size-derived typical stride.

This had a subtle bug.  Simple vector types (vec2/3/4) have no explicit
stride, so glsl_get_explicit_stride() returns 0.  This never matches the
typical stride for a vector, so is_strided_vector() would return true
for basically any vector type, causing the vectorizer to bail.

I found this by looking at a compute shader with scalar SSBO loads at
offsets 0x220, 0x224, 0x228, 0x22c.  nir_opt_load_store_vectorize would
properly vectorize the first two into a vec2 load, but would refuse to
extend it to a vec3 and ultimately vec4 load because is_strided_vector()
saw a vec2 and freaked out.

Neither ACO nor ANV do load/store vectorization before lowering derefs,
so this shouldn't affect them.  However, I'd like to fix this bug to
avoid the trap for anyone who decides to in the future.  In a branch
where anv used this lowering, this cut an additional 38% of the send
messages in the shader by properly vectorizing more things.

Reviewed-by: Rhys Perry <[email protected]>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4255>

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