[WebAssembly] made assembler parse block_type

Summary:
This was previously ignored and an incorrect value generated.

Also fixed Disassembler's handling of block_type.

Reviewers: dschuff, aheejin

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

Differential Revision: https://reviews.llvm.org/D56092

llvm-svn: 350270