Update post order number for merged SCC.

Update post order number for merged SCC.

Function loop_distribution::break_alias_scc_partitions needs to compute
SCC with runtime alias edges skipped.  As a result, partitions could be
re-assigned larger post order number than SCC's precedent partition and
distributed before the precedent one.  This fixes the issue by updating
the merged partition to the minimal post order in SCC.

gcc/
    PR tree-optimization/94125
    * tree-loop-distribution.c
    (loop_distribution::break_alias_scc_partitions): Update post order
    number for merged scc.

gcc/testsuite/
    PR tree-optimization/94125
    * gcc.dg/tree-ssa/pr94125.c: New test.