[WebAssembly] Fix WebAssemblyOptimizeReturned after r300367

Summary:
Refactoring changed paramHasAttr(1 + i) to paramHasAttr(0), fix that to
paramHasAttr(i).
Add more tests to WebAssemblyOptimizeReturned that catch that
regression.

Reviewers: dschuff

Subscribers: jfb, sbc100, llvm-commits

Differential Revision: https://reviews.llvm.org/D32136

llvm-svn: 300502