SCEVExpander::expandAddRecExprLiterally(): check before casting as Instruction

Summary:
An alternative to D48597.
Fixes [[ https://bugs.llvm.org/show_bug.cgi?id=37936 | PR37936 ]].

The problem is as follows:
1. `indvars` marks `%dec` as `NUW`.
2. `loop-instsimplify` runs `instsimplify`, which constant-folds `%dec` to -1 (D47908)
3. `loop-reduce` tries to do some further modification, but crashes
    with an type assertion in cast, because `%dec` is no longer an `Instruction`,

If the runline is split into two, i.e. you first run `-indvars -loop-instsimplify`,
store that into a file, and then run `-loop-reduce`, there is no crash.

So it looks like the problem is due to `-loop-instsimplify` not discarding SCEV.
But in this case we can just not crash if it's not an `Instruction`.
This is just a local fix, unlike D48597, so there may very well be other problems.

Reviewers: mkazantsev, uabelho, sanjoy, silviu.baranga, wmi

Reviewed By: mkazantsev

Subscribers: evstupac, javed.absar, spatel, llvm-commits

Differential Revision: https://reviews.llvm.org/D48599

llvm-svn: 335950