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[InstCombine] Avoid redundant or out-of-order debug value sinking

Authored by StephenTozer on Feb 26 2021, 5:04 AM.

Description

[InstCombine] Avoid redundant or out-of-order debug value sinking

This patch modifies TryToSinkInstruction in the InstCombine pass, to prevent
redundant debug intrinsics from being produced, and also prevent the intrinsics
from being emitted in an incorrect order. It does this by ensuring that when
this pass sinks an instruction and creates clones of the debug intrinsics that
use that instruction, it inserts those debug intrinsics in their original order,
and only inserts the last debug intrinsic for each variable in the Instruction's
block.

Differential revision: https://reviews.llvm.org/D95463