Fix single stepping over the IT instruction

The ARM instruction emulator had 2 bugs related to the handling of the
IT instruction causing an error in single stepping:
* We haven't initialized the IT mask from the CPSR so if the last
  instruction of the IT block is a branch and the condition is false
  then the emulator evaluated the branch what resulted in an incorrect
  pc for the next instruction.
* The ITSTATE was advanced before the execution of each instruction. As
  a result the emulator was using the condition of following instruction
  in every case. The ITSTATE should be edvanced after the execution of
  an instruction except after an IT instruction.

Differential revision: http://reviews.llvm.org/D16772

llvm-svn: 259509