For the current Atom processor, the fastest way to handle a call

For the current Atom processor, the fastest way to handle a call
indirect through a memory address is to load the memory address into
a register and then call indirect through the register.

This patch implements this improvement by modifying SelectionDAG to
force a function address which is a memory reference to be loaded
into a virtual register.

Patch by Sriram Murali.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178171 91177308-0d34-0410-b5e6-96231b3b80d8