[SlotIndexes] Add insertion point for insertMBBIntoMaps

Summary:
Allow the specification of an insertion point (MachineInstr)
for insertMBBIntoMaps.
This makes it possible to update slot indexes and live intervals
when trivially splitting a block by specifying the point of the
split as the insertion point for the block in the maps.

Reviewers: qcolombet, arsenm, kariddi, MaskRay, tpr

Reviewed By: kariddi

Subscribers: MatzeB, wdng, arphaman, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78417