[DFAJumpThreading] Determinator BB should precede switch-defining BB

Otherwise, it is possible that the state defined in the determinator
block defines the state for the next iteration of the loop, rather than
for the current one.

Fixes llvm-test-suite's
SingleSource/Regression/C/gcc-c-torture/execute/pr80421.c

Differential Revision: https://reviews.llvm.org/D115832