Prior to this patch, computeKnownBits would only try to deduce trailing zeros bits for getelementptrs.
This patch adds the logic to treat geps as a series of add * scaling factor.
To do so, we refactored the logic in computeKnownBitsMul to expose an API that works on a pair of KnownBits. Then, we used it in conjunction of computeKnownBitsAddSub to compute the series of adds * mul when processing geps.
Thanks to this patch, using a gep or performing an address computation directly "by hand" (ptrtoint followed by adds and mul followed by inttoptr) is going to offer the same computeKnownBits information.
Previously, the "by hand" approach would have given more information.
This is related to https://llvm.org/PR47241.