[InstCombine] Handle scalable splat in getFlippedStrictnessPredicateAndConstant (llvm#83980)

This patch adds support for canonicalization of icmp with a scalable
splat. Some optimizations assume that `icmp pred X, APInt C` is in
canonical form.

Fixes llvm#83931.

Author: dtcxzyw

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -6544,6 +6544,13 @@ InstCombiner::getFlippedStrictnessPredicateAndConstant(CmpInst::Predicate Pred,
       if (!SafeReplacementConstant)
         SafeReplacementConstant = CI;
     }
+  } else if (isa<VectorType>(C->getType())) {
+    // Handle scalable splat
+    Value *SplatC = C->getSplatValue();
+    auto *CI = dyn_cast_or_null<ConstantInt>(SplatC);
+    // Bail out if the constant can't be safely incremented/decremented.
+    if (!CI || !ConstantIsOk(CI))
+      return std::nullopt;
   } else {
     // ConstantExpr?
     return std::nullopt;

llvm/test/Transforms/InstCombine/pr83931.ll

@@ -0,0 +1,15 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 4
+; RUN: opt -S -passes=instcombine < %s | FileCheck %s
+
+define <vscale x 2 x i1> @dont_crash(<vscale x 2 x i64> %x) {
+; CHECK-LABEL: define <vscale x 2 x i1> @dont_crash(
+; CHECK-SAME: <vscale x 2 x i64> [[X:%.*]]) {
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[RET:%.*]] = icmp sgt <vscale x 2 x i64> [[X]], shufflevector (<vscale x 2 x i64> insertelement (<vscale x 2 x i64> poison, i64 -309383, i64 0), <vscale x 2 x i64> poison, <vscale x 2 x i32> zeroinitializer)
+; CHECK-NEXT:    ret <vscale x 2 x i1> [[RET]]
+;
+entry:
+  %div = sdiv <vscale x 2 x i64> %x, splat (i64 309383)
+  %ret = icmp sge <vscale x 2 x i64> %div, zeroinitializer
+  ret <vscale x 2 x i1> %ret
+}

llvm/test/Transforms/InstCombine/select.ll

@@ -3423,7 +3423,7 @@ define <vscale x 2 x i32> @scalable_sign_bits(<vscale x 2 x i8> %x) {
 define <vscale x 2 x i1> @scalable_non_zero(<vscale x 2 x i32> %x) {
 ; CHECK-LABEL: @scalable_non_zero(
 ; CHECK-NEXT:    [[A:%.*]] = or <vscale x 2 x i32> [[X:%.*]], shufflevector (<vscale x 2 x i32> insertelement (<vscale x 2 x i32> poison, i32 1, i64 0), <vscale x 2 x i32> poison, <vscale x 2 x i32> zeroinitializer)
-; CHECK-NEXT:    [[CMP:%.*]] = icmp ule <vscale x 2 x i32> [[A]], shufflevector (<vscale x 2 x i32> insertelement (<vscale x 2 x i32> poison, i32 56, i64 0), <vscale x 2 x i32> poison, <vscale x 2 x i32> zeroinitializer)
+; CHECK-NEXT:    [[CMP:%.*]] = icmp ult <vscale x 2 x i32> [[A]], shufflevector (<vscale x 2 x i32> insertelement (<vscale x 2 x i32> poison, i32 57, i64 0), <vscale x 2 x i32> poison, <vscale x 2 x i32> zeroinitializer)
 ; CHECK-NEXT:    ret <vscale x 2 x i1> [[CMP]]
 ;
   %a = or <vscale x 2 x i32> %x, splat (i32 1)

llvm/test/Transforms/InstCombine/vscale_cmp.ll

@@ -3,7 +3,7 @@
 
 define <vscale x 2 x i1> @sge(<vscale x 2 x i8> %x) {
 ; CHECK-LABEL: @sge(
-; CHECK-NEXT:    [[CMP:%.*]] = icmp sge <vscale x 2 x i8> [[X:%.*]], zeroinitializer
+; CHECK-NEXT:    [[CMP:%.*]] = icmp sgt <vscale x 2 x i8> [[X:%.*]], shufflevector (<vscale x 2 x i8> insertelement (<vscale x 2 x i8> poison, i8 -1, i64 0), <vscale x 2 x i8> poison, <vscale x 2 x i32> zeroinitializer)
 ; CHECK-NEXT:    ret <vscale x 2 x i1> [[CMP]]
 ;
   %cmp = icmp sge <vscale x 2 x i8> %x, zeroinitializer