[InstCombine] add assert and explanatory comment for fold removed in r279568; NFC

[spatel]

Summary

I deleted a fold from InstCombine at:
https://reviews.llvm.org/rL279568

because it (like any InstCombine to a constant?) should always happen in InstSimplify, however, it's not obvious what the assumptions are in the remaining code.

Add a comment and assert to make it clearer.

[spatel]

I think I missed an important set of parens in the previous draft, but got no warning from clang.

[efriedma]

I don't think the parentheses actually matter here; "(x || y) && true" is equivalent to "x || (y && true)".

LGTM.

llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp

assert(TheDiv->getOpcode() == Instruction::SDiv ||
            TheDiv->getOpcode() == Instruction::UDiv);
 
     Instruction *Res =
         foldICmpDivConstConst(Cmp, TheDiv, cast<ConstantInt>(DivCst));
     assert(Res && "This div/cst should have folded!");
     return Res;
   }
 
+  // If the comparison constant changes with the shift, the comparison cannot
+  // succeed (bits of the comparison constant cannot match the shifted value).
+  // This should be known by InstSimplify and already be folded to true/false.
+  assert(((IsAShr && C->shl(ShAmtVal).ashr(ShAmtVal) == *C) ||
+          (!IsAShr && C->shl(ShAmtVal).lshr(ShAmtVal) == *C)) &&
+         "Expected icmp+shr simplify did not occur.");
+
   // Check if the bits shifted out are known to be zero. If so, we can compare
   // against the unshifted value:
   //  (X & 4) >> 1 == 2  --> (X & 4) == 4.
   ConstantInt *ShiftedCmpRHS = Builder->getInt(*C << ShAmtVal);
   if (Shr->hasOneUse() && Shr->isExact())
     return new ICmpInst(Pred, X, ShiftedCmpRHS);
 
   if (Shr->hasOneUse()) {