Revert r307581, "Avoid doing conservative phi checks in aliasSameBasePointerGEPs() if no phis have been visited yet."

It broke stage2 tests in selfhosting.

llvm-svn: 307613
diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 704b5eb..9e042da 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -1873,7 +1873,7 @@
       if (Known.countMaxLeadingZeros() < BitWidth - ShiftVal)
         return true;
       // Are all the bits to be shifted out known zero?
-      if (Known.isUnknown() || Known.countMinTrailingZeros() >= ShiftVal)
+      if (Known.countMinTrailingZeros() >= ShiftVal)
         return isKnownNonZero(X, Depth, Q);
     }
   }