[WebAssembly] Be consistent in generating trivial test input files

For each checked-in wasm file, make sure the there is
corresponding .ll file that can be used to regenerate it
if needed.

Add test/Object/Inputs/trivial-object-test.wasm to match other
formats and add some new wasm tests in test/Object.

Differential Revision: https://reviews.llvm.org/D35213

llvm-svn: 307585