[WebAssembly] clang-format/clang-tidy AsmParser (NFC)

Summary:
- LLVM clang-format style doesn't allow one-line ifs.
- LLVM clang-tidy style says method names should start with a lowercase
  letter. But currently WebAssemblyAsmParser's parent class
  MCTargetAsmParser is mixing lowercase and uppercase method names
  itself so overridden methods cannot be renamed now.
- Changed else ifs after returns to ifs.
- Added some newlines for readability.

Reviewers: aardappel, sbc100

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

Differential Revision: https://reviews.llvm.org/D55350

llvm-svn: 348648