[WebAssembly] Emit br_table for most switch instructions

Summary:
Always convert switches to br_tables unless there is only one case,
which is equivalent to a simple branch. This reduces code size for wasm,
and we defer possible jump table optimizations to the VM.
Addresses PR41502.

Reviewers: kripken, sunfish

Subscribers: dschuff, sbc100, jgravelle-google, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60966

llvm-svn: 359038