[WebAssembly] WebAssemblyFastISel getelementptr variable index support

Summary:
Previously -fast-isel getelementptr would constant-fold non-constant i8
load/stores.

Reviewers: sunfish

Subscribers: jfb, dschuff, sbc100, llvm-commits

Differential Revision: https://reviews.llvm.org/D34044

llvm-svn: 306060