[RegAlloc] Keep a copy of live interval for the spilled vregs in HoistSpillHelper.

This is to fix PR34502. After rL311401, the live range of spilled vreg will be
cleared. HoistSpill need to use the live range of the original vreg before splitting
to know the moving range of the spills. The patch saves a copy of live interval for
the spilled vreg inside of HoistSpillHelper.

Differential Revision: https://reviews.llvm.org/D37578

llvm-svn: 313197