Speed up addRegisterDead by adding more fast checks before performing the expensive

Speed up addRegisterDead by adding more fast checks before performing the expensive
subregister query, and by increasing the size of the subregister hashtable so
that there are fewer collisions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@54781 91177308-0d34-0410-b5e6-96231b3b80d8