nir/split_vars: fixup some more explicit_stride related issues.

With vkpipelinedb Samuel discovered a regression since we stopped stripping types at the spir-v level.

This adds a check to the var splitting for the case where it asserts the type hasn't changed, when it has just created a bare type, and it's different than the original type which has an explicit stride.

This also removes a pointless assert that also triggers.

Fixes: 3b3653c4 (nir/spirv: don't use bare types, remove assert in split vars for testing)