There are a lot of complexities to handling switches efficiently:

1. Order matters for expression switches with non-constant cases and
for type expressions with interface types. We have to respect
side-effects, and we also can't allow later cases to accidentally take
precedence over earlier cases.

2. For runs of integers, floats, and string constants in expression
switches or runs of concrete types in type switches, we want to emit
efficient binary searches.

3. For runs of consecutive integers in expression switches, we want to
collapse them into range comparisons.

4. For binary searches of strings, we want to compare by length first,
because that's more efficient and we don't need to respect any
particular ordering.

5. For "switch true { ... }" and "switch false { ... }", we want to
optimize "case x:" as simply "if x" or "if !x", respectively, unless x
is interface-typed.

The current swt.go code reflects how these constraints have been
incrementally added over time, with each of them being handled ad
hocly in different parts of the code. Also, the existing code tries
very hard to reuse logic between expression and type switches, even
though the similarities are very superficial.

This CL rewrites switch handling to better abstract away the logic
involved in constructing the binary searches. In particular, it's
intended to make further optimizations to switch dispatch much easier.

It also eliminates the need for both OXCASE and OCASE ops, and a
subsequent CL can collapse the two.

Passes toolstash-check.

Change-Id: Ifcd1e56f81f858117a412971d82e98abe7c4481f
Reviewed-on: https://go-review.googlesource.com/c/go/+/194660
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>