[WebAssembly] Inline accessor functions that are called only once.

Looks like these accessor functions are a bit overly defensive, and
due to the amount of code, that part isn't easy to read. We have
code to log translation results in writeTo and writeRelocations, so
I don't think we need to log it again in these functions.
I think the new function is much easier to understand.

Differential Revision: https://reviews.llvm.org/D43713

llvm-svn: 326277