[SCEV] Guard movement of insertion point for loop-invariants

r320789 suppressed moving the insertion point of SCEV expressions with
dev/rem operations to the loop header in non-loop-invariant situations.
This, and similar, hoisting is also unsafe in the loop-invariant case,
since there may be a guard against a zero denominator. This is an
adjustment to the fix of r320789 to suppress the movement even in the
loop-invariant case.

This fixes PR30806.

Differential Revision: https://reviews.llvm.org/D54713

llvm-svn: 347934