[WebAssembly] Disable MachineBlockPlacement pass

Summary:
This pass hurts code size for wasm and sometimes generates irreducible
control flow.
Context: https://github.com/emscripten-core/emscripten/pull/8233

Reviewers: kripken, dschuff

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58953

llvm-svn: 355437