cmd/compile: simplify bad conversion check
cmd/compile: simplify bad conversion check

Now that we're using OCONVIDATA(x) everywhere we formerly used
OIDATA(OCONVIFACE(x)), there should be no OCONVIFACE operations that
take a shape type.

Change-Id: I4fb056456c60481c6dfe7bc111fca6223567e6a8
Reviewed-on: https://go-review.googlesource.com/c/go/+/344577
Trust: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>
Trust: Dan Scales <7338a4f274e3bf1354488b9d1f344f4c9a69a8fb@google.com>
Run-TryBot: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dan Scales <7338a4f274e3bf1354488b9d1f344f4c9a69a8fb@google.com>

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