In order to help LLVM disambiguate accesses to the COMMON
block variables, this patch creates a TBAA sub-tree for each
COMMON block, and the places all variables belonging to this
COMMON block into this sub-tree. The structure looks like this:
```
common /blk/ a, b, c
"global data"
|
|- "blk_"
|
|- "blk_/bytes_0_to_3"
|- "blk_/bytes_4_to_7"
|- "blk_/bytes_8_to_11"
```
The TBAA tag for "a" is created in "blk_/bytes_0_to_3" root, etc.
The byte spans are created based on the `storage` information
provided by `fir.declare` (#155742).
See also:
https://discourse.llvm.org/t/rfc-flang-representation-for-objects-inside-physical-storage/88026
556ff19db9