InstSimplify: Optimize using dividend in sdiv

[majnemer]

Summary

The dividend in an sdiv tells us the largest and smallest possible
results. Use this fact to optimize comparisons against an sdiv with a
constant dividend.

[majnemer]

Address review comments.

[majnemer]

Closed by commit rL208999 (authored by @majnemer).

lib/Analysis/InstructionSimplify.cpp

if (ConstantInt *CI = dyn_cast<ConstantInt>(RHS)) {
     } else if (match(LHS, m_UDiv(m_ConstantInt(CI2), m_Value()))) {
       // 'udiv CI2, x' produces [0, CI2].
       Upper = CI2->getValue() + 1;
     } else if (match(LHS, m_UDiv(m_Value(), m_ConstantInt(CI2)))) {
       // 'udiv x, CI2' produces [0, UINT_MAX / CI2].
       APInt NegOne = APInt::getAllOnesValue(Width);
       if (!CI2->isZero())
         Upper = NegOne.udiv(CI2->getValue()) + 1;
+    } else if (match(LHS, m_SDiv(m_ConstantInt(CI2), m_Value()))) {
+      // 'sdiv CI2, x' produces [-|CI2|, |CI2|].
+      Upper = CI2->getValue().abs() + 1;
+      Lower = (-Upper) + 1;
     } else if (match(LHS, m_SDiv(m_Value(), m_ConstantInt(CI2)))) {
       // 'sdiv x, CI2' produces [INT_MIN / CI2, INT_MAX / CI2].
       APInt IntMin = APInt::getSignedMinValue(Width);
       APInt IntMax = APInt::getSignedMaxValue(Width);
       APInt Val = CI2->getValue().abs();
       if (!Val.isMinValue()) {
         Lower = IntMin.sdiv(Val);
         Upper = IntMax.sdiv(Val) + 1;

test/Transforms/InstSimplify/compare.ll

define i1 @compare_always_false_ne(i16 %a) {
   %1 = zext i16 %a to i32
   %2 = sub nsw i32 0, %1
   %3 = icmp ne i32 %2, 1
   ret i1 %3
 
 ; CHECK-LABEL: @compare_always_false_ne
 ; CHECK-NEXT: ret i1 true
 }
+
+define i1 @compare_dividend(i32 %a) {
+  %div = sdiv i32 2, %a
+  %cmp = icmp eq i32 %div, 3
+  ret i1 %cmp
+
+; CHECK-LABEL: @compare_dividend
+; CHECK-NEXT: ret i1 false
+}