Tighten up the splice() API for bundled instructions.

Tighten up the splice() API for bundled instructions.

Remove the instr_iterator versions of the splice() functions. It doesn't
seem useful to be able to splice sequences of instructions that don't
consist of full bundles.

The normal splice functions that take MBB::iterator arguments are not
changed, and they can move whole bundles around without any problems.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@170456 91177308-0d34-0410-b5e6-96231b3b80d8