[InstCombine] prevent possible miscompile with negate+sdiv of vector op

// 0 - (X sdiv C)  -> (X sdiv -C)  provided the negation doesn't overflow.

This fold has been around for many years and nobody noticed the potential
vector miscompile from overflow until recently...
So it seems unlikely that there's much demand for a vector sdiv optimization
on arbitrary vector constants, so just limit the matching to splat constants
to avoid the possible bug.

Differential Revision: https://reviews.llvm.org/D60426

llvm-svn: 358005