Don't skip splitSeparateComponents in eliminateDeadDefs for HoistSpillHelper::hoistAllSpills.

Because HoistSpillHelper::hoistAllSpills is called in postOptimization, before the
patch we didn't want LiveRangeEdit::eliminateDeadDefs to call splitSeparateComponents
and generate unassigned new vregs. However, skipping splitSeparateComponents will make
verify-machineinstrs unhappy, so I remove the early return, and use
HoistSpillHelper::LRE_DidCloneVirtReg to assign physreg/stackslot for those new vregs.

In addition, some code reorganization to make class HoistSpillHelper privately inheriting
from LiveRangeEdit::Delegate possible. This is to be consistent with class RAGreedy and
class RegisterCoalescer.

Differential Revision: http://reviews.llvm.org/D19142

llvm-svn: 266489