compiler/types: Don't place members in the previous substruct's end padding

With the following structures :

  struct StructA
  {
     uint64_t value0;
     uint8_t  value1;
  };

  struct TopStruct
  {
     struct StructA a;
     uint8_t value3;
  };

Currently offsetof(struct TopStruct, value3) = 9. While the same code
on the CPU gives offsetof(struct TopStruct, value3) = 16.

This is impacting OpenCL kernels we're trying to use to build
acceleration structures.

v2: Add comment/link to some description of the alignment/size
    computation

Cc: mesa-stable
Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@collabora.com>
Part-of: <!16940>
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