[InstCombine] Remove redundant/bogus mul_with_overflow combines
[InstCombine] Remove redundant/bogus mul_with_overflow combines

As pointed out in D60518 folding mulo(%x, undef) to {undef, undef}
isn't correct. As a correct version of this already exists in
InstructionSimplify (https://github.com/llvm-mirror/llvm/blob/bd8056ef326e075cc500f3f0cfcd1193bc200594/lib/Analysis/InstructionSimplify.cpp#L4750-L4757) this is just
dead code though. Drop it together with the mul(%x, 0) -> {0, false}
fold that is also already handled by InstSimplify.

Differential Revision: https://reviews.llvm.org/D60649

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@358339 91177308-0d34-0410-b5e6-96231b3b80d8

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