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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@300798 91177308-0d34-0410-b5e6-96231b3b80d8