[APInt] In operator!, handle single word case by comparing VAL to 0 directly and handle multiword case by comparing countLeadingZerosSlowCase() to BitWidth.

We were using operator=(0) which implicitly calls countLeadingZeros but only to compare with 64 to determine if we can compare VAL or pVal[0] to uint64_t. By handling the multiword case with countLeadingZerosSlowCase==BitWidth we can prevent a load of pVal[0] from being inserted inline at each call site. This saves a little bit of code size.

llvm-svn: 301842

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