[WebAssembly][FastISel] Do not assume naive CmpInst lowering

Summary:
Fixes https://bugs.llvm.org/show_bug.cgi?id=40172. See
test/CodeGen/WebAssembly/PR40172.ll for an explanation.

Reviewers: dschuff, aheejin

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

Differential Revision: https://reviews.llvm.org/D56457

llvm-svn: 351127