Bug 470132 - s390x: Fix the wrap-around case in VGM

Bug 470132 - s390x: Fix the wrap-around case in VGM

Valgrind's implementation of VGM is incomplete:

* It doesn't support generating a wrap-around bit mask.  Such a mask
  should result when the ending bit position is smaller than the starting
  bit position.  Valgrind runs into an assertion failure instead.

* It doesn't ignore unused bits in the I2 and I3 fields of the
  instruction, as it should.

Fix this by re-implementing the main logic in s390_irgen_VGM().