[SCEV] identical instructions don't compute equal values

Before this change `HasSameValue` would return true for distinct
`alloca` instructions if they happened to be allocating the same
type (`alloca` instructions are not specified as reading memory).  This
change adds an explicit whitelist of instruction types for which
"identical" instructions compute the same value.

Fixes PR24952.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@248690 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed