Mark llvm::ConstantExpr::getAsInstruction as const

Summary:
getAsInstruction is the only non-const member method.
It is impossible to enforce const-correctness because of it.

Reviewers: jmolloy, majnemer

Reviewed By: jmolloy

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70113