[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.

Includes fix for the nullptr deref that caused the original commit
to be reverted in 9d630297.

Fixes : PR38815

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