[MachineOutliner] NFC: Add debug output to MachineOutliner::outline

Add some debug output to `outline` to assist in debugging + understanding the
code.

This will say

- How many things we found worth turning into outlined functions
- Whether or not candidates were pruned via the outlining algorithm
- The function created (if it was created)
- Where the calls were inserted
- What instruction was used to create the call

Sample output below:

```
NUMBER OF POTENTIAL FUNCTIONS: 5
WALKING FUNCTION LIST
PRUNED: 0/2 candidates
OUTLINE: Expected benefit (12 B) > threshold (1 B)
NEW FUNCTION: OUTLINED_FUNCTION_0
CREATE OUTLINED CALLS
  CALL: OUTLINED_FUNCTION_0 in bar:<unknown>
   .. BL @OUTLINED_FUNCTION_0, implicit-def $lr, implicit $sp
  CALL: OUTLINED_FUNCTION_0 in bar:<unknown>
   .. BL @OUTLINED_FUNCTION_0, implicit-def $lr, implicit $sp
PRUNED: 2/2 candidates
SKIP: Expected benefit (0 B) < threshold (1 B)
PRUNED: 0/2 candidates
OUTLINE: Expected benefit (8 B) > threshold (1 B)
NEW FUNCTION: OUTLINED_FUNCTION_1
CREATE OUTLINED CALLS
  CALL: OUTLINED_FUNCTION_1 in bar:<unknown>
   .. BL @OUTLINED_FUNCTION_1, implicit-def $lr, implicit $sp
  CALL: OUTLINED_FUNCTION_1 in bar:<unknown>
   .. BL @OUTLINED_FUNCTION_1, implicit-def $lr, implicit $sp
PRUNED: 2/2 candidates
SKIP: Expected benefit (0 B) < threshold (1 B)
PRUNED: 2/2 candidates
SKIP: Expected benefit (0 B) < threshold (1 B)
```