Make switch-to-lookup-large-types.ll more reliable

When larger integer types are natively supported simplifycfg will use an
inline constant instead of a global variable for this transform. I noticed
this while trying to automatically infer the datalayout from the target
triple in opt if it is not explicitly specified. Since the x86_64
datalayout includes "n8:16:32:64", this test started failing.

While touching this file also change i128 to i64 in the first test since
this was intended behaviour in the original commit.

Reviewed By: spatel, fhahn

Differential Revision: https://reviews.llvm.org/D141055