[dev.unified] test: relax panic message expectations
[dev.unified] test: relax panic message expectations

In this test, traditionally the comparison `*l == r[0]` was left as a
comparison between `*l` (type `any`) and `r[0]` (type `*int`), and the
rest of the compiler needed to handle mixed-typed comparisons.
However, this means more complexity for wiring up explicit rtypes.

To simplify rtype handling, the next CL will change unified IR to
instead handle the expression as `*l == any(r[0])`. However, a
consequence of this currently is that walk will now sequence the
`any(r[0])` expression first, because it involves a
concrete-to-interface conversion. And in turn, this means the `r[0]`
panic ("index out of bounds") will take priority over the `*l`
panic ("nil pointer dereference").

This is a change in user-visible semantics in some cases, but the Go
spec leaves this unspecified, so it shouldn't be an issue. Note also:
gccgo has the same behavior (i.e., panicking on index out of bounds,
not nil pointer dereference), and cmd/compile also already has the
same behavior when the interface conversion is explicit (as in the
added "nil pointer dereference #3" test case).

Updates #23735.
Updates #32187.

Change-Id: I49e5dcca85b4680f9c8780ef0013e64254d38fe5
Reviewed-on: https://go-review.googlesource.com/c/go/+/418097
Run-TryBot: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Cuong Manh Le <54b4457f1dff6823379654eb5867596497ba782f@gmail.com>
Reviewed-by: David Chase <ae171625adc8645b8b79b7e405a6da5aba5341e0@google.com>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go