When looking for a spill slot in reg scavenger, find one that matches RC

When looking for an available spill slot, the register scavenger would stop
after finding the first one with no register assigned to it. That slot may
have size and alignment that do not meet the requirements of the register
that is to be spilled. Instead, find an available slot that is the closest
in size and alignment to one that is needed to spill a register from RC.

Differential Revision: http://reviews.llvm.org/D20295

llvm-svn: 269969