[XLA:GPU] Remove a map data structure used to support the code generation of

GetTupleElement inside a fused computation.

The map initially records the fusion parameters and their base addresses. Then
during the forward traversal of the fused computation that produces elemental
ir generators, we generate code to calculate the base address for the
GetTupleElement instructions that produce tuple results, and add the
information to the map. As a result, this map records all the tuple producing
instructions whose results can be used in GetTupleElement instructions. During
code generation, if a GetTupleElement instruction produces a non-tuple result,
we look up the map for the base pointer of the tuple to generate code that
produces the instruction result. On the other hand, if a GetTupleELement
instruction produces a tuple result, we do not generate any code as the code
for such instructions is already generated when we produce the map data
structure.

Such a map data structure is not necessary. The base addresses for a parameter
can be derived from the IrArray of the parameter, which can be looked up
through the parameter number. We change the elemental ir emitter for
GetTupleElement instructions to return the base address for the tuple producing
GetTupleElement instructions to avoid looking up the map for such instructions.
In doing that, we also avoid generating code for tuple producing
GetTupleElement instructions duing the phase that produces elemental ir
generators to clean up the code generation for GetTupleElement instructions.

PiperOrigin-RevId: 218922219