[APInt] Add an isNullValue method to check for all bits being zero. Use it in...

[APInt] Add an isNullValue method to check for all bits being zero. Use it in a couple internal methods where it makes more sense than isMinValue or !getBoolValue. NFC

I used Null rather than Zero to match the getNullValue method name.

There are some other places outside APInt where isNullValue would be more readable than isMinValue even though they do the same thing. I'll update those in future patches.

llvm-svn: 301695