SSA names have one associated variable, their SSA_NAME_VAR. The
ability is added to on demand adding further variables to an SSA name
via a bitmap that is stored in a hashtable mapping SSA name version
to a bitmap of variable UIDs that at some point in the program give
a name to the SSA names value.
Throughout optimization the variable mapping of a SSA name changes
in the following occasions:
- The SSA name is deleted. In this case the variable mapping is
deleted as well.
- The SSA name is copied to another SSA name. This relates both
SSA names and their variable mapping becomes the union of both
variable mappings.