Add a new pre-allocation pass to assign adjacent registers for Neon instructions

Add a new pre-allocation pass to assign adjacent registers for Neon instructions
that have that constraint.  This is currently just assigning a fixed set of
registers, and it only handles VLDn for n=2,3,4 with DPR registers.
I'm going to expand it to handle more operations next; we can make it smarter
once everything is working correctly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@78256 91177308-0d34-0410-b5e6-96231b3b80d8