[MachineInstrBuilder] Support for adding a ConstantPoolIndex MO with an additional offset.

MachineInstrBuilder::addDisp can already add an immediate or global address MO with an adjusted offset, this patch adds support for constant pool indices as well.

All remaining MO types still assert - there are a number of other types that could support adjusted offsets but I have no test cases at this time.

Required to fix a regression in D13988 found by Mikael Holmén during stress testing (test case attached).

Differential Revision: http://reviews.llvm.org/D14867

llvm-svn: 253795