AArch64: refactor table-driven NEON lookup.

This extracts the table-driven intrinsic lookup phase into a separate
function, to be used by EmitCommonNeonBuiltinExpr soon.

It also simplifies the logic used in that lookup, since VectorCastArgN
and ScalarArgN were actually identical.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201847 91177308-0d34-0410-b5e6-96231b3b80d8