X86: Tighten up test.

llc CPU autodection bites again. Speculative fix for bot failures.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205940 91177308-0d34-0410-b5e6-96231b3b80d8
Fix to support properly cleaning up failed address sinking against constants

As it turns out the source of the sunkaddr can be a constant, in which case
there is not an instruction to delete, causing the cleanup code introduced in
r204833 to crash. This patch adds a dynamic check to ensure the deleted value is
in fact an instruction and not a constant.

Patch by Louis Gerbarg <lgg@apple.com>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205941 91177308-0d34-0410-b5e6-96231b3b80d8