[InstCombine] Remove redundant splats in InstCombineVectorOps

Splatting the first vector element of the result of a BinOp, where any of the
BinOp's operands are the result of a first vector element splat can be simplified to
splatting the first vector element of the result of the BinOp

Differential Revision: https://reviews.llvm.org/D135876