Don't use an expensive check for two-address-ness when we have the information sittin...

Don't use an expensive check for two-address-ness when we have the information sitting around to determine it much more quickly,
This speeds up the local register allocator from 0.37s to 0.31s on instcombine.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@53359 91177308-0d34-0410-b5e6-96231b3b80d8