[BasicAA] Remove misleading overflow check

GEP decomposition currently checks whether the multiplication of
the linear expression offset and GEP scale overflows. However, if
everything else works correctly, this overflow check is both
unnecessary and dangerously misleading. While it will avoid an
overflow in Scale * Offset in particular, other parts of the
calculation (including those on dynamic values) may still overflow.
The code working on the decomposed GEPs is responsible for ensuring
that it remains correct in the presence of overflow. D112611 fixes
the last issue of that kind that I'm aware of (in fact, the overflow
check was originally introduced to work around precisely that issue).

Differential Revision: https://reviews.llvm.org/D112618

Author: nikic

llvm/lib/Analysis/BasicAliasAnalysis.cpp

@@ -657,23 +657,8 @@ BasicAAResult::DecomposeGEPExpression(const Value *V, const DataLayout &DL,
 
       // The GEP index scale ("Scale") scales C1*V+C2, yielding (C1*V+C2)*Scale.
       // This gives us an aggregate computation of (C1*Scale)*V + C2*Scale.
-
-      // It can be the case that, even through C1*V+C2 does not overflow for
-      // relevant values of V, (C2*Scale) can overflow. In that case, we cannot
-      // decompose the expression in this way.
-      //
-      // FIXME: C1*Scale and the other operations in the decomposed
-      // (C1*Scale)*V+C2*Scale can also overflow. We should check for this
-      // possibility.
-      bool Overflow;
-      APInt ScaledOffset = LE.Offset.sextOrTrunc(MaxPointerSize)
-                           .smul_ov(Scale, Overflow);
-      if (Overflow) {
-        LE = LinearExpression(CastedValue(Index, 0, SExtBits, TruncBits));
-      } else {
-        Decomposed.Offset += ScaledOffset;
-        Scale *= LE.Scale.sextOrTrunc(MaxPointerSize);
-      }
+      Decomposed.Offset += LE.Offset.sextOrTrunc(MaxPointerSize) * Scale;
+      Scale *= LE.Scale.sextOrTrunc(MaxPointerSize);
 
       // If we already had an occurrence of this index variable, merge this
       // scale into it.  For example, we want to handle: