[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

llvm-svn: 344107