[CVP] Use LVI to constant fold deopt operands

Deopt operands are generally intended to record information about a site in code with minimal perturbation of the surrounding code. Idiomatically, they also tend to appear down rare paths. Putting these together, we have an obvious case for extending CVP w/deopt operand constant folding. Arguably, we should be doing this for all operands on all instructions, but that's definitely a much larger and risky change.

Differential Revision: https://reviews.llvm.org/D55678

llvm-svn: 351774