Remove casts between Value** and Constant**, which won't work if a

Remove casts between Value** and Constant**, which won't work if a
static_cast from Constant* to Value* has to adjust the "this" pointer.
This is groundwork for PR889.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@123435 91177308-0d34-0410-b5e6-96231b3b80d8