[APInt] Move operator&=(uint64_t) inline and use memset to clear the upper words.

[APInt] Move operator&=(uint64_t) inline and use memset to clear the upper words.

This method is pretty new and probably isn't use much in the code base so this should have a negligible size impact. The OR and XOR operators are already inline.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@298870 91177308-0d34-0410-b5e6-96231b3b80d8