[WebAssembly] Fix signature parsing for 'try' in AsmParser

Summary:
Like `block` or `loop`, `try` can take an optional signature which can
be omitted. This patch allows `try`'s signature to be omitted. Also
added some tests for EH instructions.

Reviewers: aardappel

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

Differential Revision: https://reviews.llvm.org/D53873

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@345888 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed