[WebAssembly] Refactor load ISel tablegen patterns into classes

Not all of these will be able to be used by atomics because tablegen, but it
still seems like a good change by itself.

Differential Revision: https://reviews.llvm.org/D37345

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@312287 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed