Fixed a problem in IRForTarget where we would not
delete a constant after we replaced it with a
dynamically-computed value.  Also ensured that we
replace all users of the constant if there are
multiple ones.  Added a testcase.

<rdar://problem/14379043>


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@186363 91177308-0d34-0410-b5e6-96231b3b80d8