Add support for spreading register allocation.

Add support for spreading register allocation.

Add a -linearscan-skip-count argument (default to 0) that tells the
allocator to remember the last N registers it allocated and skip them
when looking for a register candidate.  This tends to spread out
register usage and free up post-allocation scheduling at the cost of
slightly more register pressure.  The primary benefit is the ability
to backschedule reloads.

This is turned off by default.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@89356 91177308-0d34-0410-b5e6-96231b3b80d8