[WebAssembly] Support constant offsets on loads and stores

[WebAssembly] Support constant offsets on loads and stores

This is just prototype for load/store for i32 types. I'll add them to
the rest of the types if we like this direction.

Differential Revision: http://reviews.llvm.org/D15197

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@254807 91177308-0d34-0410-b5e6-96231b3b80d8