[WebAssembly] Only write 32-bits for WebAssembly::OPERAND_OFFSET32

A bug was found where an offset of -1 would generate an encoding
of max int64 which is invalid in the binary format.

Differential Revision: https://reviews.llvm.org/D45280

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@329238 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed