[CodeGenPrepare] Zap the argument of llvm.assume when deleting it

We know that the argument is mostly likely dead, so we can purge it
early. Otherwise it would make it to codegen, and can block further
optimizations.

Author: d0k

llvm/lib/CodeGen/CodeGenPrepare.cpp

@@ -2044,7 +2044,12 @@ bool CodeGenPrepare::optimizeCallInst(CallInst *CI, bool &ModifiedDT) {
     switch (II->getIntrinsicID()) {
     default: break;
     case Intrinsic::assume: {
+      Value *Operand = II->getOperand(0);
       II->eraseFromParent();
+      // Prune the operand, it's most likely dead.
+      RecursivelyDeleteTriviallyDeadInstructions(
+          Operand, TLInfo, nullptr,
+          [&](Value *V) { removeAllAssertingVHReferences(V); });
       return true;
     }
 

llvm/test/Transforms/CodeGenPrepare/X86/delete-assume-dead-code.ll

@@ -0,0 +1,35 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -codegenprepare -S -mtriple=x86_64-linux < %s | FileCheck %s
+
+define i32 @test1(i8* %d) nounwind {
+; CHECK-LABEL: @test1(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[L:%.*]] = load i8, i8* [[D:%.*]], align 1
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i8 [[L]], 0
+; CHECK-NEXT:    br i1 [[CMP]], label [[EXIT:%.*]], label [[IF_END:%.*]]
+; CHECK:       if.end:
+; CHECK-NEXT:    br label [[EXIT]]
+; CHECK:       exit:
+; CHECK-NEXT:    [[TMP0:%.*]] = icmp eq i8 [[L]], 0
+; CHECK-NEXT:    [[CONV:%.*]] = zext i1 [[TMP0]] to i32
+; CHECK-NEXT:    ret i32 [[CONV]]
+;
+entry:
+  %l = load i8, i8* %d
+  %cmp = icmp eq i8 %l, 0
+  br i1 %cmp, label %exit, label %if.end
+
+if.end:
+  %gep = getelementptr i8, i8* %d, i32 42
+  %call = call i64 @foo(i8* %gep) nounwind readonly
+  %cmp2 = icmp ne i64 %call, 0
+  call void @llvm.assume(i1 %cmp2)
+  br label %exit
+
+exit:
+  %conv = zext i1 %cmp to i32
+  ret i32 %conv
+}
+
+declare i64 @foo(i8*) nounwind readonly
+declare void @llvm.assume(i1 noundef) nounwind willreturn