llvm-6502/lib/Target/Sparc

Meta TODO list:
1. Create a new DAG -> DAG instruction selector, by adding patterns to the
   instructions.
2. ???
3. profit!

To-do
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* Keep the address of the constant pool in a register instead of forming its
  address all of the time.
* We can fold small constant offsets into the %hi/%lo references to constant
  pool addresses as well.