[MLIR] Fix AsmPrinter.cpp for single ssa-id AffineMap

The AsmPrinter wrongly assumes that all single ssa-id AffineMap
are the identity map for the purpose of printing.
This CL adds the missing level of indirection as well as a test.

This bug was originally shaken off by the experimental TC->MLIR path.

Before this CL, the test would print:
```
mlfunc @mlfuncsimplemap(%arg0 : affineint, %arg1 : affineint, %arg2 : affineint) {
  for %i0 = 0 to %arg0 {
    for %i1 = 0 to %i0 {
                   ~~~   should be %arg1
      %c42_i32 = constant 42 : i32
    }
  }
  return
}
```

PiperOrigin-RevId: 214120817