[TableGen][GlobalISel] Allow duplicate RendererFns

Allow different GICustomOperandRenderers to use the same RendererFn.
This avoids the need for targets to define a bunch of identical C++
renderer functions with different names.

Without this fix TableGen would have emitted code that tried to define
the GICR enumeration with duplicate enumerators.

Differential Revision: https://reviews.llvm.org/D96587