[WebAssembly] Encode block signatures as SLEB instead of ULEB

Use SLEB (varint) for block_type immediates in accordance with the spec.

Patch by Yury Delendik

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@300490 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed