[Local] Simplify the alignment limits in getOrEnforceKnownAlignment. NFCI

[craig.topper]

Summary

We previously clamped the trailing zero count to 31 bits. And
then clamped the final alignment to MaximumAlignment which is
1 << 29.

This patch simplifies this to just clamp the trailing zero to
29 using MaxAlignmentExponent.

I was looking into changin this function to use Align/MaybeAlign
and noticed this.

[lebedev.ri]

LG

llvm/lib/Transforms/Utils/Local.cpp

unsigned llvm::getOrEnforceKnownAlignment(Value *V, unsigned PrefAlign,
   assert(V->getType()->isPointerTy() &&
          "getOrEnforceKnownAlignment expects a pointer!");
 
   KnownBits Known = computeKnownBits(V, DL, 0, AC, CxtI, DT);
   unsigned TrailZ = Known.countMinTrailingZeros();
 
   // Avoid trouble with ridiculously large TrailZ values, such as
   // those computed from a null pointer.
-  TrailZ = std::min(TrailZ, unsigned(sizeof(unsigned) * CHAR_BIT - 1));
+  // LLVM doesn't support alignments larger than (1 << MaxAlignmentExponent).
+  TrailZ = std::min(TrailZ, Value::MaxAlignmentExponent);
 
   unsigned Align = 1u << std::min(Known.getBitWidth() - 1, TrailZ);
 
-  // LLVM doesn't support alignments larger than this currently.
-  Align = std::min(Align, +Value::MaximumAlignment);
-
   if (PrefAlign > Align)
     Align = enforceKnownAlignment(V, Align, PrefAlign, DL);
 
   // We don't need to make any adjustment.
   return Align;
 }
 
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