regalloc: Make live range splitting deterministic
regalloc: Make live range splitting deterministic

Summary:
The order of traversal of `mode_and_insn` determines the order of the
instructions we end up inserting. This diff makes `mode_and_insn` an ordered
map so that our output is deterministic.

Reviewed By: minjang

Differential Revision: D5600378

fbshipit-source-id: b06e7b50a0b6a3d004de4a47581cf88f0b080655

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