[WebAssembly] Fix fneg lowering

Summary:
Subtraction from zero and floating point negation do not have the same
semantics, so fix lowering.

Reviewers: aheejin, dschuff

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

Differential Revision: https://reviews.llvm.org/D52948

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@344107 91177308-0d34-0410-b5e6-96231b3b80d8