WebAssembly: basic instructions todo, and basic register info.

Summary:
This code is based on AArch64 for modern backend good practice, and NVPTX for
virtual ISA concerns.

Reviewers: sunfish

Subscribers: aemerson, llvm-commits, jfb

Differential Revision: http://reviews.llvm.org/D11070

llvm-svn: 241923
16 files changed