[PM] Simplify (ha! ha!) the way that instcombine calls the

[PM] Simplify (ha! ha!) the way that instcombine calls the
SimplifyLibCalls utility by sinking it into the specific call part of
the combiner.

This will avoid us needing to do any contortions to build this object in
a subsequent refactoring I'm doing and seems generally better factored.
We don't need this utility everywhere and it carries no interesting
state so we might as well build it on demand.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226654 91177308-0d34-0410-b5e6-96231b3b80d8