[dev.typeparams] cmd/compile: add "toolstash -cmp"-like test of -d=unified
[dev.typeparams] cmd/compile: add "toolstash -cmp"-like test of -d=unified

This CL adds a longtest test to make sure -d=unified=1 produces output
identical to -d=unified=0.

Change-Id: I2c5d38f67dbc8fecd8332a91ba7cae22225b090c
Reviewed-on: https://go-review.googlesource.com/c/go/+/329429
Trust: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>
Run-TryBot: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Cuong Manh Le <54b4457f1dff6823379654eb5867596497ba782f@gmail.com>

golang/go,golang/go,CAFxX/go,golang/go,CAFxX/go,CAFxX/go,CAFxX/go,nawawi/go,nawawi/go,nawawi/go,CAFxX/go,cloudflare/go,cloudflare/go,nawawi/go,golang/go,golang/go,cloudflare/go,CAFxX/go,golang/go,cloudflare/go,cloudflare/go,nawawi/go,cloudflare/go,nawawi/go,cloudflare/go,nawawi/go,CAFxX/go,CAFxX/go,cloudflare/go,golang/go,golang/go,nawawi/go