[InlineSpiller] simplify insertReload() NFC

Summary:
The repeated use of std::next() on a MachineBasicBlock::iterator was
clever, but we only need to reconstruct the iterator post creation of
the spill instruction.

This helps simplifying where we plan to place the spill, as discussed in
D77849.

From here, we can simplify the code a little by flipping the return code
of a helper.

Reviewers: efriedma

Reviewed By: efriedma

Subscribers: qcolombet, hiraditya, llvm-commits, srhines

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78520