cmd/compile: when GOSSAFUNC is set, dump the current pass on crash

When an SSA pass ICEs, it calls f.Fatalf, which terminates the
compiler. When GOSSAFUNC is set, the current pass is not written
to ssa.html. This CL makes it write ssa.html when it calls Fatalf,
for the ease of debugging.

Change-Id: I5d55e4258f0693d89c48c0a84984f2f893b0811d
Reviewed-on: https://go-review.googlesource.com/c/go/+/307509


Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: David Chase <drchase@google.com>