[AMDGPU] LRO: allow same-BB non-lookthrough users for PHI

Loop headers frequently consume the loop-carried value in the header
block via non-lookthrough ops (e.g. byte-wise vector binops). LRO’s
same-BB filter currently prunes these users, so the loop-carried PHI
is not coerced to i32 and the intended packed form is lost.

Relax the filter: when the def is a PHI, allow same-BB non-lookthrough
users. Also fix the check to look at the user (CII) rather than the
def (II) so the walk does not terminate prematurely.

Author: michaelselehov

llvm/lib/Target/AMDGPU/AMDGPULateCodeGenPrepare.cpp

@@ -150,7 +150,10 @@ class LiveRegOptimizer {
       if (!CVisited.insert(CII).second)
         continue;
 
-      if (CII->getParent() == II->getParent() && !IsLookThru(II))
+      // Same-BB filter must look at the *user*; and allow non-lookthrough
+      // users when the def is a PHI (loop-header pattern).
+      if (CII->getParent() == II->getParent() && !IsLookThru(CII) &&
+          !isa<PHINode>(II))
         continue;
 
       if (isOpLegal(CII))