[IR] Add Instruction::getInsertionPointAfterDef()

Transforms occasionally want to insert an instruction directly
after the definition point of a value. This involves quite a few
different edge cases, e.g. for phi nodes the next insertion point
is not the next instruction, and for invokes and callbrs its not
even in the same block. Additionally, the insertion point may not
exist at all if catchswitch is involved.

This adds a general Instruction::getInsertionPointAfterDef() API to
implement the necessary logic. For now it is used in two places
where this should be mostly NFC. I will follow up with additional
uses where this fixes specific bugs in the existing implementations.

Differential Revision: https://reviews.llvm.org/D129660