[Attributor] Provide a generic interface to check live instructions

Summary:
Similar to `Attributor::checkForAllCallSites`, we now provide such
functionality for instructions of a certain opcode through
`Attributor::checkForAllInstructions` and the convenient wrapper
`Attributor::checkForAllCallLikeInstructions`. This cleans up code,
avoids duplication, and simplifies the usage of liveness information.

Reviewers: sstefan1, uenoku

Subscribers: hiraditya, bollu, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65731

llvm-svn: 367961