[ValueTracking] Prevent a call to computeKnownBits if we already know the state of the bit we would calculate. Also reuse a temporary APInt instead of creating a new one.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@300239 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed