Change APFloat::convertFromInteger to take the incoming

Change APFloat::convertFromInteger to take the incoming
bit width instead of number of words allocated, which
makes it actually work for int->APF conversions.
Adjust callers.  Add const to one of the APInt constructors
to prevent surprising match when called with const
argument.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@42210 91177308-0d34-0410-b5e6-96231b3b80d8