[AVR] Disable register coalescing to the PTRDISPREGS class

[AVR] Disable register coalescing to the PTRDISPREGS class

If we would allow register coalescing on PTRDISPREGS class then register
allocator can lock Z register to some virtual register. Larger instructions
requiring a memory acces then fail during the register allocation phase since
there is no available register to hold a pointer if Y register was already
taken for a stack frame. This patch prevents it by keeping Z register
spillable. It does it by not allowing coalescer to lock it.

Original discussion on https://github.com/avr-rust/rust/issues/128.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@362298 91177308-0d34-0410-b5e6-96231b3b80d8