[SLP] Improve gather tree nodes matching when users are PHIs. (#70111)

This is re-commit of #69392 and also fixes issue #69670 which was
uncovered with the prior commit.
For delayed gather emission it may be incorrect to use stab instruction
as insertion point if it is a PHI operand. For that case insertion point
is adjusted to be at the end of block, ensuring that prior dependecy
vector code is emitted earlier.