[APInt] Move operator=(uint64_t) inline as its pretty simple and is often used with...

[APInt] Move operator=(uint64_t) inline as its pretty simple and is often used with small constants that the compiler can optimize.

While there recognize that we only need to clearUnusedBits on the single word case.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@298881 91177308-0d34-0410-b5e6-96231b3b80d8