[SLP]Fix PR58177: Improve isUndefVector function to avoid extra freeze.

[SLP]Fix PR58177: Improve isUndefVector function to avoid extra freeze.

Freeze instruction in some cases makes codegen worse, so need to be very
careful when emitting it. Instead improve analysis in isUndefVector
function to generate mask of unused elements and use it in the analysis.

Differential Revision: https://reviews.llvm.org/D135382