[InstSimplify] use knownbits to fold more udiv/urem

We could use knownbits on both operands for even more folds (and there are
already tests in place for that), but this is enough to recover the example
from:
https://github.com/llvm/llvm-project/issues/51934
(the tests are derived from the code in that example)

I am assuming no noticeable compile-time impact from this because udiv/urem
are rare opcodes.

Differential Revision: https://reviews.llvm.org/D116616

llvm/lib/Analysis/InstructionSimplify.cpp

@@ -1079,6 +1079,16 @@ static bool isDivZero(Value *X, Value *Y, const SimplifyQuery &Q,
   }
 
   // IsSigned == false.
+
+  // Is the unsigned dividend known to be less than a constant divisor?
+  // TODO: Convert this (and above) to range analysis
+  //      ("computeConstantRangeIncludingKnownBits")?
+  const APInt *C;
+  if (match(Y, m_APInt(C)) &&
+      computeKnownBits(X, Q.DL, 0, Q.AC, Q.CxtI, Q.DT).getMaxValue().ult(*C))
+    return true;
+
+  // Try again for any divisor:
   // Is the dividend unsigned less than the divisor?
   return isICmpTrue(ICmpInst::ICMP_ULT, X, Y, Q, MaxRecurse);
 }

llvm/test/Transforms/InstSimplify/div.ll

@@ -155,10 +155,7 @@ define i32 @not_udiv_constant_dividend_known_smaller_than_divisor(i32 %x) {
 
 define i8 @udiv_dividend_known_smaller_than_constant_divisor2(i1 %b) {
 ; CHECK-LABEL: @udiv_dividend_known_smaller_than_constant_divisor2(
-; CHECK-NEXT:    [[T0:%.*]] = zext i1 [[B:%.*]] to i8
+; CHECK-NEXT:    ret i8 0
-; CHECK-NEXT:    [[XOR:%.*]] = xor i8 [[T0]], 12
-; CHECK-NEXT:    [[R:%.*]] = udiv i8 [[XOR]], 14
-; CHECK-NEXT:    ret i8 [[R]]
 ;
   %t0 = zext i1 %b to i8
   %xor = xor i8 %t0, 12
@@ -166,6 +163,8 @@ define i8 @udiv_dividend_known_smaller_than_constant_divisor2(i1 %b) {
   ret i8 %r
 }
 
+; negative test - dividend can equal 13
+
 define i8 @not_udiv_dividend_known_smaller_than_constant_divisor2(i1 %b) {
 ; CHECK-LABEL: @not_udiv_dividend_known_smaller_than_constant_divisor2(
 ; CHECK-NEXT:    [[T0:%.*]] = zext i1 [[B:%.*]] to i8

llvm/test/Transforms/InstSimplify/rem.ll

@@ -187,8 +187,7 @@ define i8 @urem_dividend_known_smaller_than_constant_divisor2(i1 %b) {
 ; CHECK-LABEL: @urem_dividend_known_smaller_than_constant_divisor2(
 ; CHECK-NEXT:    [[T0:%.*]] = zext i1 [[B:%.*]] to i8
 ; CHECK-NEXT:    [[XOR:%.*]] = xor i8 [[T0]], 12
-; CHECK-NEXT:    [[R:%.*]] = urem i8 [[XOR]], 14
+; CHECK-NEXT:    ret i8 [[XOR]]
-; CHECK-NEXT:    ret i8 [[R]]
 ;
   %t0 = zext i1 %b to i8
   %xor = xor i8 %t0, 12
@@ -196,6 +195,8 @@ define i8 @urem_dividend_known_smaller_than_constant_divisor2(i1 %b) {
   ret i8 %r
 }
 
+; negative test - dividend can equal 13
+
 define i8 @not_urem_dividend_known_smaller_than_constant_divisor2(i1 %b) {
 ; CHECK-LABEL: @not_urem_dividend_known_smaller_than_constant_divisor2(
 ; CHECK-NEXT:    [[T0:%.*]] = zext i1 [[B:%.*]] to i8