[DebugInfo] Avoid re-ordering assignments in LCSSA

[DebugInfo] Avoid re-ordering assignments in LCSSA

The LCSSA pass makes use of a function insertDebugValuesForPHIs() to
propogate dbg.value() intrinsics to newly inserted PHI instructions. Faulty
behaviour occurs when the parent PHI of a newly inserted PHI is not the
most recent assignment to a source variable. insertDebugValuesForPHIs ends
up propagating a value that isn't the most recent assignemnt.

This change removes the call to insertDebugValuesForPHIs() from LCSSA,
preventing incorrect dbg.value intrinsics from being propagated.
Propagating variable locations between blocks will occur later, during
LiveDebugValues.

Differential Revision: https://reviews.llvm.org/D92576