[ValueTracking] Use APInt instead of auto. NFC

This is a pre-commit for a patch I'm working on to turn KnownZero/One into a struct. Once I do that the type here will be less obvious.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301324 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed