[APInt] Don't call getActiveBits() in ult/ugt(uint64_t) if its a single word.

The compiled code already needs to check single/multi word for the countLeadingZeros call inside of getActiveBits, but it isn't able to optimize out the leadingZeros call in the single word case that can't produce a value larger than 64.

This shrank the opt binary by about 5-6k on my local x86-64 build.

llvm-svn: 300798