[ConstantHoisting] Remove dupliate logic in constant hoisting

Summary:
As metioned in https://reviews.llvm.org/D34576, checkings in
`collectConstantCandidates` can be replaced by using
`llvm::canReplaceOperandWithVariable`.

The only special case is that `collectConstantCandidates` return false for
all `IntrinsicInst` but it is safe for us to collect constant candidates from
`IntrinsicInst`.

Reviewers: pirama, efriedma, srhines

Reviewed By: efriedma

Subscribers: llvm-commits, javed.absar

Differential Revision: https://reviews.llvm.org/D34921

llvm-svn: 307587