This is useful for debugging line number assignment and
also for making sense of pathological line number inputs.

Activated with
-gcflags=-d=ssa/number_lines/stats=1 (the bit matters)
-gcflags=-d=ssa/number_lines/debug

Stats:
"SUM_LINE_RANGE",
   SUM for f in files {MAX line in f {line}-MIN line in f {line}}
"MAXMIN_LINE_RANGE",
   MAX for f in files {MAX line in f {line}} -
   MIN for f in files {MIN line in f {line}}
"MAXFILE", maxfile,
   MAX for f in files {f}
"NFILES", len(entries)
   | files |

Change-Id: I8a7336e6370452fe2e3a62de17606db9bd6a6fd3
Reviewed-on: https://go-review.googlesource.com/c/go/+/174947
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>