[GlobalISel] Permit select() to erase.

[GlobalISel] Permit select() to erase.

Erasing reverse_iterators is problematic; iterate manually.
While there, keep track of the range of inserted instructions.
It can miss instructions inserted elsewhere, but those are harder
to track.

Differential Revision: http://reviews.llvm.org/D22924

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@286272 91177308-0d34-0410-b5e6-96231b3b80d8