Merging r370404:

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r370404 | rksimon | 2019-08-29 22:22:08 +0200 (Thu, 29 Aug 2019) | 3 lines

[X86][SSE] combinePMULDQ - pmuldq(x, 0) -> zero vector (PR43159)

ISD::isBuildVectorAllZeros permits undef elements to be present, which means we can't return it as a zero vector. PMULDQ/PMULUDQ is an extending multiply so a multiply by zero of the lower 32-bits should result in a zero 64-bit element.
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llvm-svn: 370445