[LSV] Don't assume that bitcast ops are Instructions.

Summary:
When we ask the builder to create a bitcast on a constant, we get back a
constant, not an instruction.

Reviewers: asbirlea

Subscribers: jholewinski, mzolotukhin, llvm-commits, arsenm

Differential Revision: https://reviews.llvm.org/D22878

llvm-svn: 276922