[SLP] Rename IsUniformStride to IsUnitStride. NFCI

IsUniformStride is only used when the stride is a unit-stride, i.e. in a
plain wide vector load. This tightens the condition and renames it to
isUnitStride. It removes the old unused getUniformStrided() variant, as
isUnitStride should now imply that the stride is known.

Reviewed By: vdmitrie, ABataev

Differential Revision: https://reviews.llvm.org/D150662