[SCEV] Suppress hoisting insertion point of binops when unsafe

InsertBinop tries to move insertion-points out of loops for expressions
that are loop-invariant. This patch adds a new parameter, IsSafeToHost,
to guard that hoisting. This allows callers to suppress that hoisting
for unsafe situations, such as divisions that may have a zero
denominator.

This fixes PR38697.

Differential Revision: https://reviews.llvm.org/D55232

llvm-svn: 360280