[dev.regabi] cmd/compile: fix ICE due to large uint64 constants

It's an error to call Int64Val on constants that don't fit into
int64. CL 272654 made the compiler stricter about detecting misuse,
and revealed that we were using it improperly in detecting consecutive
integer-switch cases. That particular usage actually did work in
practice, but it's easy and best to just fix it.

Fixes #43480.

Change-Id: I56f722d75e83091638ac43b80e45df0b0ad7d48d
Reviewed-on: https://go-review.googlesource.com/c/go/+/281272


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>