[ValueTracking] Use BitWidth local variable instead of re-reading it from KnownZero. NFC

This is a pre-commit for a patch that I'm working on to merge KnownZero/KnownOne into a KnownBits struct which would have had to touch this line.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301323 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed