When converting to integer, do bit manipulations in the destination

When converting to integer, do bit manipulations in the destination
memory rather than in a copy of the APFloat.  This avoids problems
when the destination is wider than our significand and is cleaner.

Also provide deterministic values in all cases where conversion
fails, namely zero for NaNs and the minimal or maximal value
respectively for underflow or overflow.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@43626 91177308-0d34-0410-b5e6-96231b3b80d8