TableGen: Support Intrinsic values in SearchableTable

Summary:
We will use this in the AMDGPU backend in a subsequent patch
in the stack to lookup target-specific per-intrinsic information.

The generic CodeGenIntrinsic machinery is used to ensure that,
even though we don't calculate actual enum values here, we do
get the intrinsics in the right order for the binary search
index.

Change-Id: If61cd5587963a4c5a1cc53df1e59c5e4dec1f9dc

Reviewers: arsenm, rampitec, b-sumner

Subscribers: wdng, tpr, llvm-commits

Differential Revision: https://reviews.llvm.org/D44935

llvm-svn: 328937