WebAssembly: start instructions

Summary:
* Add 64-bit address space feature.
* Rename SIMD feature to SIMD128.
* Handle single-thread model with an IR pass (same way ARM does).
* Rename generic processor to MVP, to follow design's lead.
* Add bleeding-edge processors, with all features included.
* Fix a few DEBUG_TYPE to match other backends.

Test Plan: ninja check

Reviewers: sunfish

Subscribers: jfb, llvm-commits

Differential Revision: http://reviews.llvm.org/D10880

llvm-svn: 241211