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[DebugInfo] Improve dbg preservation in LSR.

Authored by markus on Mon, Oct 5, 12:27 AM.

Description

[DebugInfo] Improve dbg preservation in LSR.

Use SCEV to salvage additional @llvm.dbg.value that have turned into
referencing undef after transformation (and traditional
salvageDebugInfo). Before transformation compute SCEV for each
@llvm.dbg.value in the loop body and store it (along side its current
DIExpression). After transformation update those @llvm.dbg.value now
referencing undef by comparing its stored SCEV to the SCEV of the
current loop-header PHI-nodes. Allow match with offset by inserting
compensation code in the DIExpression.

Fixes : PR38815

Differential Revision: https://reviews.llvm.org/D87494