Add an early implementation of a partial inlining pass. The idea behind this

Add an early implementation of a partial inlining pass. The idea behind this
is that, for functions whose bodies are entirely guarded by an if-statement, it
can be profitable to pull the test out of the callee and into the caller.

This code has had some cursory testing, but still has a number of known issues
on the LLVM test suite.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@73338 91177308-0d34-0410-b5e6-96231b3b80d8