Avoid hoisting spills when looking at a copy from another register that is also

Avoid hoisting spills when looking at a copy from another register that is also
about to be spilled.

This can only happen when two extra snippet registers are included in the spill,
and there is a copy between them. Hoisting the spill creates problems because
the hoist will mark the copy for later dead code elimination, and spilling the
second register will turn the copy into a spill.

<rdar://problem/9420853>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@131192 91177308-0d34-0410-b5e6-96231b3b80d8