cmd/compile: remove references to *os.File from ssa package

cmd/compile: remove references to *os.File from ssa package

This reduces the size of the ssa export data
by 10%, from 76154 to 67886.

It doesn't appear that #20084, which would do this automatically,
is going to be fixed soon. Do it manually for now.

This speeds up compiling cmd/compile/internal/amd64
and presumably its comrades as well:

name          old time/op       new time/op       delta
CompileAMD64       89.6ms ± 6%       86.7ms ± 5%  -3.29%  (p=0.000 n=49+47)

name          old user-time/op  new user-time/op  delta
CompileAMD64        116ms ± 5%        112ms ± 5%  -3.51%  (p=0.000 n=45+42)

name          old alloc/op      new alloc/op      delta
CompileAMD64       26.7MB ± 0%       25.8MB ± 0%  -3.26%  (p=0.008 n=5+5)

name          old allocs/op     new allocs/op     delta
CompileAMD64         223k ± 0%         213k ± 0%  -4.46%  (p=0.008 n=5+5)

Updates #20084

Change-Id: I49e8951c5bfce63ad2b7f4fc3bfa0868c53114f9
Reviewed-on: https://go-review.googlesource.com/41493
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>