spirv: Add error checking for Block and BufferBlock decorations

spirv: Add error checking for Block and BufferBlock decorations

Variable pointers being well-defined across the block boundary requires
a couple of very specific SPIR-V validation rules.  Normally, we'd trust
the validator to catch these but since CTS tests have been found in the
wild which violate them, we'll carry our own checks.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>