[SelectionDAG] Tidyup computeKnownBits extension/truncation cases. NFCI.

We don't need to extend/truncate the Known structure before calling computeKnownBits - it will reset at the start of the function.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316962 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed