[OPENMP] Fix for PR34398: assert with random access iterator if the

step>1.

If the loop is a loot with random access iterators and the iteration
construct is represented it += n, then the compiler crashed because of
reusing of the same MaterializedTemporaryExpr around N. Patch fixes it
by using the expression as written, without any special kind of
wrappings.

llvm-svn: 312292