[WebAssembly] Fixed Disassembler ignoring endian swap on big endian.

Summary: This fixes: https://bugs.llvm.org/show_bug.cgi?id=40620

Reviewers: aheejin

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D57933

llvm-svn: 353496