X86: Tighten up test.

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