Allow X86 QBE to be extended

Enhancements and updates to allow X86Mir2LIR Backend to be subclassed
for experimentation.  Add virtual in a whole bunch of places, and make
some other changes to get this to work.

Change-Id: I0980a19bc5d5725f91660f98c95f1f51c17ee9b6
Signed-off-by: Mark Mendell <mark.p.mendell@intel.com>
10 files changed