[WebAssembly] Make rethrow instruction take a target BB argument

Summary:
This patch changes the rethrow instruction to take a BB argument in LLVM
backend, like `br` and `br_if`s. This BB is a target catch BB the
rethrow instruction unwinds to. This BB argument will be converted to an
relative depth immediate at the end of CFGStackify pass, as in the same
way of branches.

RETHROW_TO_CALLER is a codegen-only instruction that should be used when
a rethrow instruction does not have an unwind destination BB, i.e., it
should rethrow to its caller function.

Reviewers: dschuff

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

Differential Revision: https://reviews.llvm.org/D48260

llvm-svn: 334998