Move the GlobalBaseReg field out of X86ISelDAGToDAG.cpp

Move the GlobalBaseReg field out of X86ISelDAGToDAG.cpp
and X86FastISel.cpp into X86MachineFunction.h, so that it
can be shared, instead of having each selector keep track
of its own.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@56825 91177308-0d34-0410-b5e6-96231b3b80d8