[APInt] Use trailing bit counting methods instead of population count method...

[APInt] Use trailing bit counting methods instead of population count method in isAllOnesValue, isMaxSigendValue, and isMinSignedValue. NFCI

The trailing bit methods will early out if they find a bit of the opposite while popcount must always look at all bits. I also assume that more CPUs implement trailing bit counting with native instructions than population count.

llvm-svn: 306154