[WebAssembly] Make starting indices calcaulation simpler (NFC)

Summary:
At the beginning of `assignIndexes() function, when `FunctionIndex` and
`GlobalIndex` variables are created, `InputFunctions` and `InputGlobals`
vectors are guaranteed to be empty, because those vectors are only
populated in `assignIndexes()` function. Current code looks like they
are nonempty, so this patch deletes them for better readability.

Reviewers: sbc100

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

Differential Revision: https://reviews.llvm.org/D54687

llvm-svn: 347272