cmd/compile: improve the help message of go tool compile -d ssa/help

[dlespiau]

I noticed it was somewhat difficult to read the ssa/help output:

  $ go tool compile -d ssa/help
  compile: GcFlag -d=ssa/<phase>/<flag>[=<value>]|[:<function_name>]
  <phase> is one of:
  check, all, build, intrinsics, early_phielim, early_copyelim
  early_deadcode, short_circuit, decompose_user, opt, zero_arg_cse
  opt_deadcode, generic_cse, phiopt, nilcheckelim, prove, loopbce
  decompose_builtin, dec, late_opt, generic_deadcode, check_bce
  writebarrier, fuse, dse, insert_resched_checks, tighten, lower
  lowered_cse, lowered_deadcode, checkLower, late_phielim, late_copyelim
  phi_tighten, late_deadcode, critical, likelyadjust, layout, schedule
  late_nilcheck, flagalloc, regalloc, stackframe, trim
  <flag> is one of on, off, debug, mem, time, test, stats, dump
  <value> defaults to 1
  <function_name> is required for "dump", specifies name of function to dump after <phase>
  Except for dump, output is directed to standard out; dump appears in a file.
  Phase "all" supports flags "time", "mem", and "dump".
  Phases "intrinsics" supports flags "on", "off", and "debug".
  Interpretation of the "debug" value depends on the phase.
  Dump files are named <phase>__<function_name>_<seq>.dump.

With a few tweaks here and there it looks more pleasing to the eye:

  $ go tool compile -d ssa/help
  compile: usage: -d=ssa/<phase>/<flag>[=<value>|<function_name>]

  <phase> is one of:

  	check, all, build, intrinsics, early_phielim, early_copyelim, early_deadcode
  	short_circuit, decompose_user, opt, zero_arg_cse, opt_deadcode, generic_cse
  	phiopt, nilcheckelim, prove, loopbce, decompose_builtin, dec, late_opt
  	generic_deadcode, check_bce, writebarrier, fuse, dse, insert_resched_checks
  	tighten, lower, lowered_cse, lowered_deadcode, checkLower, late_phielim
  	late_copyelim, phi_tighten, late_deadcode, critical, likelyadjust, layout
  	schedule, late_nilcheck, flagalloc, regalloc, stackframe, trim

  <flag> is one of on, off, debug, mem, time, test, stats, dump.
  Phase all supports flags time, mem, and dump.
  Phase intrinsics supports flags on, off, and debug.

  <value> defaults to 1.

  <function_name> is required for dump, specifies name of function to dump after <phase>.

  Except for dump, output is directed to standard out; dump appears in a file.
  Dump files are named <phase>__<function_name>_<seq>.dump.

[gopherbot]

CL https://golang.org/cl/41470 mentions this issue.

[bradfitz]

I consider this documentation and fine for Go 1.9 if you can find a reviewer. Probably @randall77 or @josharian.

[josharian]

I'll review. Please consider adding a few examples of commonly used flags.