Add pass to promote sret.

This pass transforms 

  %struct._Point = type { i32, i32, i32, i32, i32, i32 }
  define internal void @foo(%struct._Point* sret  %agg.result)

into

  %struct._Point = type { i32, i32, i32, i32, i32, i32 }
  define internal %struct._Point @foo()

This pass updates foo() clients appropriately to use
getresult instruction to extract return values.

This pass is not yet ready for prime time.

llvm-svn: 47776