[WebAssembly] CFG stackify support for exception handling

Summary:
This adds support for exception handling to CFGStackify pass. This only
adds TRY / END_TRY markers and DOES NOT yet fix unwind mismatches that
can be created by the linearization of the CFG into the structural wasm
format. The mismatch fix will be added by following patches.

In detail, this patch
- Added support for TRY / END_TRY markers to support EH
- Changed many static functions into class member functions as they take
too many arguments now
- Added several more bookeeping data structures
- Refactored routines that decide where to insert markers, because
without refactoring this got too complicated as we added support for new
kinds of markers (TRY/END_TRY).
- Rewrote rethrow instructions' BB arguments to relative depths in EH
pad stack.

Reviewers: dschuff, sunfish

Subscribers: sbc100, jgravelle-google, llvm-commits

Differential Revision: https://reviews.llvm.org/D48273

llvm-svn: 339967