[InstCombine] canonicalize insert+splat to/from element 0 of vector

We recognize a splat from element 0 in (VectorUtils) llvm::getSplatValue()
and also in ShuffleVectorInst::isZeroEltSplatMask(), so this converts
to that form for better matching.

The backend generically turns these patterns into build_vector,
so there should be no codegen difference.

llvm-svn: 365342