[MachinePipeliner] Fix risky iterator usage R++, --R

[MachinePipeliner] Fix risky iterator usage R++, --R

When we calculate MII, we use two loops, one with iterator R++ to
check whether we can reserve the resource, then --R to move back
the iterator to do reservation.

This is risky, as R++, --R may not point to the same element at all.
The can cause wrong MII.

Differential Revision: https://reviews.llvm.org/D63536

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@364353 91177308-0d34-0410-b5e6-96231b3b80d8