Panics are only distinguished by their type and line number, so
if we can trigger two of those panics in the same line, use the
same panic call. For example, in a[i]+b[j] we need only one
panicindex call that both bounds checks can use.
Change-Id: Ia2b6d3b1a67f2775df05fb72b8a1b149833572b7
Reviewed-on: https://go-review.googlesource.com/16772
Run-TryBot: Keith Randall <khr@golang.org> Reviewed-by: David Chase <drchase@google.com>