Fix inserting instructions before last in bundle.

Fix inserting instructions before last in bundle.

The builder inserts from before the insert point,
not after, so this would insert before the last
instruction in the bundle instead of after it.

I'm not sure if this can actually be a problem
with any of the current insertions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189285 91177308-0d34-0410-b5e6-96231b3b80d8