convertFromInteger, as originally written, expected sign-extended

convertFromInteger, as originally written, expected sign-extended
input.  APInt unfortunately zero-extends signed integers, so Dale
modified the function to expect zero-extended input.  Make this
assumption explicit in the function name.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@42732 91177308-0d34-0410-b5e6-96231b3b80d8