[ARM] Don't form "ands" when it isn't scheduled correctly.

In r322972/r323136, the iteration here was changed to catch cases at the
beginning of a basic block... but we accidentally deleted an important
safety check.  Restore that check to the way it was.

Fixes https://bugs.llvm.org/show_bug.cgi?id=41116

Differential Revision: https://reviews.llvm.org/D59680

llvm-svn: 356809