[APInt] Add a isOneValue method that can determine if a number is 1 by only...

[APInt] Add a isOneValue method that can determine if a number is 1 by only using getActiveBits/countLeadingZeros

Previously you would have to use operator==(uint64_t) which does the getActiveBits call and a uint64_t comparison. But we can get all we need to know from the getActiveBits call.

This method will be used in another commit.

llvm-svn: 304854