Address-mode folding for X86FastISel. It's pretty basic, but it

catches a fair number of common cases. Note that this currently
causes Fast-ISel to leave behind lots of dead instructions.
Those will be dealt with in subsequent commits.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@56320 91177308-0d34-0410-b5e6-96231b3b80d8