[OpenMP][LLVM] Update alloca IP after `PrivCB` in `OMPIRBUIlder` (#93920)

Fixes a crash uncovered by
[pr77666.f90](https://github.com/llvm/llvm-test-suite/blob/main/Fortran/gfortran/regression/gomp/pr77666.f90)
in the test suite when delayed privatization is enabled by default.

In particular, whenever `PrivCB` (the callback responsible for
generating privatizaiton logic for an OMP variable) generates a
multi-block privatization region, the insertion point diverges: the BB
component of the IP can become a different BB from the parent block of
the instruction iterator component of the IP. This PR updates the IP to
make sure that the BB is the parent block of the instruction iterator.