Add pass to promote sret.

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@47776 91177308-0d34-0410-b5e6-96231b3b80d8