Change CostTable model to be global to all targets

Moving the X86CostTable to a common place, so that other back-ends
can share the code. Also simplifying it a bit and commoning up
tables with one and two types on operations.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172658 91177308-0d34-0410-b5e6-96231b3b80d8