[ThinLTO] Add option to limit importing based on instruction count

[ThinLTO] Add option to limit importing based on instruction count

Add a simple initial heuristic to control importing based on the number
of instructions recorded in the function's summary. Add option to
control the limit, and test using option.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@254036 91177308-0d34-0410-b5e6-96231b3b80d8