ARM/VIXL: Fix TypeConversion of large long constants to int.

The ARM VIXL backend was using Int32ConstantFrom() to get
the value of a HLongConstant for TypeConversion to int.
That function has DCHECK()s that the constant fits into a
32-bit int which is bogus in this particular case.

This was out of sync with the old ARM backend, so bring the
two in sync by using the approach from the old backend.

Bug: 37236345
Test: 646-checker-lon-const-to-int
Change-Id: I675ebd7bbaa14d5dfc3f280f62867d342cbab683