[SPIR-V] Ensure that internal intrinsic functions "ptrcast" for PHI's operand...

[SPIR-V] Ensure that internal intrinsic functions "ptrcast" for PHI's operand are inserted at the correct positions (#92536)

The goal of the PR is to ensure that newly inserted `ptrcast` internal
intrinsic functions are inserted at the correct positions, and don't
break rules of instruction domination and PHI nodes grouping at top of
basic block.