[ValueTracking] Replace all uses of ComputeSignBit with computeKnownBits.

This patch finishes off the conversion of ComputeSignBit to computeKnownBits.

Differential Revision: https://reviews.llvm.org/D33166

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@303035 91177308-0d34-0410-b5e6-96231b3b80d8
12 files changed