[DAGCombiner] Support all-ones/all-zeros SPLAT_VECTOR in more combines

This patch adds incrementally-better support for SPLAT_VECTOR in a
handful of vector combines by changing a few more
isBuildVectorAllOnes/isBuildVectorAllZeros to the equivalent
isConstantSplatVectorAllOnes/Zeros calls.

Reviewed By: paulwalker-arm

Differential Revision: https://reviews.llvm.org/D100851