Bump llvm-pretty{-bc-parser} submodules, adapt to ValPoison

This bumps the `llvm-pretty` submodule to bring in the following PRs:

* GaloisInc/llvm-pretty#86 (support LLVM 11 `IsDefault` fields in metadata)
* GaloisInc/llvm-pretty#87 (Relax the Symbol requirement to `ConstBlockAddr`)
* GaloisInc/llvm-pretty#88 (add LLVM12 poison value)
* GaloisInc/llvm-pretty#90 (Support structs with bitfields in `Text.LLVM.DebugUtils`)

This also bumps the `llvm-pretty-bc-parser` submodule to bring in the
following PRs:

* GaloisInc/llvm-pretty-bc-parser#171 (don't consume fast math flag entry)
* GaloisInc/llvm-pretty-bc-parser#169 (make `getBitString` assumptions explicit)
* GaloisInc/llvm-pretty-bc-parser#168 (support for LLVM 11 `IsDefault` fields in metadata)
* GaloisInc/llvm-pretty-bc-parser#170 (remove `Aligned` from `SubWord`)
* GaloisInc/llvm-pretty-bc-parser#172 (fix BLOCKADDRESS parsing)
* GaloisInc/llvm-pretty-bc-parser#173 (parse LLVM12 poison value)
* GaloisInc/llvm-pretty-bc-parser#178 (use Seq over list in PartialModule)

Of these, the only PR that requires code changes in Crucible is
GaloisInc/llvm-pretty#88 (add LLVM12 poison value), as this adds a new
`ValPoison` constructor to `Value'`. Properly handling LLVM poison is a large
task that I do not wish to tackle here (see #366 for discussion of how one
might do this). For now, this PR does the bare minimum:

* In `Lang.Crucible.LLVM.Translation.BlockInfo.useVal`, treat `ValPoison`
  as not referencing any identifiers.
* The other two potential sites where `ValPoison` could be matched on are in
  `Lang.Crucible.LLVM.Translation.Constant.transConstant'` and
  `Lang.Crucible.LLVM.Translation.Expr.transValue`. Since we are not handling
  `ValPoison` for now, both of these functions will simply error if they
  encounter `ValPoison`.
* I have added a section to `crucible-llvm`'s `limitations.md` document which
  describes the above limitations of poison handling.

crucible-llvm/doc/limitations.md

@@ -101,3 +101,20 @@ memory yield arbitrary values. Since LLVM often passes loaded values
 from uninitialized memory to `freeze` to ensure that the result is not
 `undef`, this will ensure that a sizable number of use cases for
 `freeze` will be handled as expected.
+
+LLVM poison limitations
+=======================
+`crucible-llvm` has limited support for tracking
+[poison values](https://releases.llvm.org/13.0.0/docs/LangRef.html#poisonvalues).
+Certain LLVM instructions and intrinsics can return poison values under
+certain circumstances, and `crucible-llvm` makes an effort to generate failing
+assertions whenever such poison values are returned. For instance, LLVM's
+`add` instruction can return poison if the result overflows, which
+`crucible-llvm` is able to detect and simulate by throwing an appropriate
+assertion failure.
+
+There are other ways to create and propagate poison that `crucible-llvm` is
+unable to track, however. There is no support for LLVM's `poison` constant
+values, and `crucible-llvm` will throw an error if it encounters such a
+`poison` constant. LLVM also permits values where only certain bits of the
+value are poison, but `crucible-llvm` is unable to reason about this.

crucible-llvm/src/Lang/Crucible/LLVM/Translation/BlockInfo.hs

@@ -284,6 +284,7 @@ useVal v = Set.unions $ case v of
   L.ValLabel _ -> []
   L.ValZeroInit -> []
   L.ValAsm{} -> [] -- TODO! inline asm ...
+  L.ValPoison{} -> []
 
   -- NB! metadata values are not considered as part of our use analysis
   L.ValMd _md -> []