[WebAssembly] Make sure event-section XFAILs for build options

rL346840 temporarily marked event-section.ll as XFAIL because it was
failing for builds with LLVM_ENABLE_EXPENSIVE_CHECKS turned on, but
to make sure it XFAILs even without LLVM_ENABLE_EXPENSIVE_CHECKS on we
need this `-verify-machineinstrs` flag, which was missing in the
previous commit.

llvm-svn: 346845