Count references to interference cache entries.
Each InterferenceCache::Cursor instance references a cache entry. A
non-zero reference count guarantees that the entry won't be reused for a
new register.
This makes it possible to have multiple live cursors examining
interference for different physregs.
The total number of live cursors into a cache must be kept below
InterferenceCache::getMaxCursors().
Code generation should be unaffected by this change, and it doesn't seem
to affect the cache replacement strategy either.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@135121
91177308-0d34-0410-b5e6-
96231b3b80d8