Simplify some C-style loops with range statements, and move some
declarations closer to their uses.

While at it, ensure that all the SymbolType consts are typed.

Change-Id: I04b06afb2c1fb249ef8093a0c5cca0a597d1e05c
Reviewed-on: https://go-review.googlesource.com/105217
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>