[APInt] Remove references to integerPartWidth and integerPart outside of APFloat implentation.

Turns out integerPartWidth only explicitly defines the width of the tc functions in the APInt class. Functions that aren't used by APInt implementation itself. Many places in the code base already assume APInt is made up of 64-bit pieces. Explicitly assuming 64-bit here doesn't make that situation much worse. A full audit would need to be done if it ever changes.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@299058 91177308-0d34-0410-b5e6-96231b3b80d8