Fix MachineSink to be able to sink instructions that use physical registers

Fix MachineSink to be able to sink instructions that use physical registers
which have no defs anywhere in the function. In particular, this fixes sinking
of instructions that reference RIP on x86-64, which is currently being modeled
as a register.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@82815 91177308-0d34-0410-b5e6-96231b3b80d8