Teach instsimplify to gracefully degrade in the presence of instructions

not attched to a basic block or function. There are conservatively
correct answers in these cases, and this makes the analysis more useful
in contexts where we have a partially formed bit of IR.

I don't have any way to test this directly... suggestions welcome here,
but I'm not seeing anything sadly. I only found this using a subsequent
patch to the inliner which runs instsimplify on partially inlined
instructions, and even then only on a quite large program. I never got
a reasonable testcase out of it, and anything I do get is likely to be
quite fragile due to requiring an interaction of two different passes,
and the only result being a segfault if it goes wrong.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@153176 91177308-0d34-0410-b5e6-96231b3b80d8

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