[X86] Don't compute known bits twice for the same SDValue in LowerMUL.

[X86] Don't compute known bits twice for the same SDValue in LowerMUL.

We called MaskedValueIsZero with two different masks, but underneath that calls computeKnownBits before applying the mask. This means we compute the same known bits twice due to the two calls. Instead just call computeKnownBits directly and apply the two masks ourselves.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@327251 91177308-0d34-0410-b5e6-96231b3b80d8