[ScopBuilder] Exclude ignored intrinsics from explicit instruction list.

Ignored intrinsics are ignored at code generation, therefore do not
need to be part of the instruction list.

Specifically, llvm.lifetime.* intrinisics are removed before code
generation, referencing them would cause a use-after-free error.

Contributed-by: Nandini Singhal <cs15mtech01004@iith.ac.in>

Differential Revision: https://reviews.llvm.org/D33768

llvm-svn: 304483