[GISel]: Provide standard interface to observe changes in GISel passes

[GISel]: Provide standard interface to observe changes in GISel passes

https://reviews.llvm.org/D54980

This provides a standard API across GISel passes to observe and notify
passes about changes (insertions/deletions/mutations) to MachineInstrs.
This patch also removes the recordInsertion method in MachineIRBuilder
and instead provides method to setObserver.

Reviewed by: vkeles.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@348406 91177308-0d34-0410-b5e6-96231b3b80d8