[WebAssembly] Autogenerate checks in simd-offset.ll

Implementing new functionality tested in this file requires adding new
tests for many IR addressing patterns, which can be a large
maintenance burden. This patch makes adding tests easier by switching
to using autogenerated checks. This patch also removes the testing
mode that has simd128 disabled because it would produce very large
checks and is not particularly interesting.

Differential Revision: https://reviews.llvm.org/D84288

Author: tlively