bug: drreg restoration can overwrite changed values in inline instrumentation

[ndrewh]

Currently we before we commit the new register state we do something like this:

    drreg_unreserve_register(drcontext, bb, instr, op1_reg);
    drreg_unreserve_register(drcontext, bb, instr, op2_reg);
    drreg_restore_app_value(drcontext, bb, instr, op1_reg, op1_reg, false);
    drreg_restore_app_value(drcontext, bb, instr, op2_reg, op2_reg, false);

    exprbuilder_commit(builder, bb, instr, scratch_ptr_reg);

    drreg_unreserve_register(drcontext, bb, instr, scratch_ptr_reg);
    drreg_restore_app_value(drcontext, bb, instr, scratch_ptr_reg, scratch_ptr_reg, false);

The intention of this sequence is to ensure that the drreg restoration doesn't get inserted after we write the new register values. But nothing about unreserve or restore prevents drreg from later inserting another restoration. We should test this -- maybe we need to pass stateful=true to drreg_restore_app_value?