cmd/compile: panic if trying to alias an intrinsic with no definitions
cmd/compile: panic if trying to alias an intrinsic with no definitions

Currently if we try to alias an intrinsic which hasn't been defined for
any architecture (such as by accidentally creating the alias before the
intrinsic is created with addF), then we'll just silently not apply any
intrinsics to those aliases.

Catch this particular case by panicking in alias if we try to apply the
alias and it did nothing.

Change-Id: I98e75fc3f7206b08fc9267cedb8db3e109ec4f5d
Reviewed-on: https://go-review.googlesource.com/c/go/+/224637
Run-TryBot: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>
Reviewed-by: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

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