Factor out the code for determining when symblic addresses

Factor out the code for determining when symblic addresses
require RIP-relative addressing and use it to fix a bug
in X86FastISel in x86-64 PIC mode, where it was trying to
use base/index registers with RIP-relative addresses. This
fixes a bunch of x86-64 testsuite failures.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@56676 91177308-0d34-0410-b5e6-96231b3b80d8