[AArch64][GlobalISel] Add legalization for G_VECREDUCE_MUL (#68398)

Author: ChuongGoh-arm

llvm/lib/Target/AArch64/GISel/AArch64LegalizerInfo.cpp

@@ -895,6 +895,13 @@ AArch64LegalizerInfo::AArch64LegalizerInfo(const AArch64Subtarget &ST)
       .clampMaxNumElements(1, s16, 8)
       .lower();
 
+  getActionDefinitionsBuilder(G_VECREDUCE_MUL)
+      .clampMaxNumElements(1, s32, 2)
+      .clampMaxNumElements(1, s16, 4)
+      .clampMaxNumElements(1, s8, 8)
+      .scalarize(1)
+      .lower();
+
   getActionDefinitionsBuilder(
       {G_VECREDUCE_OR, G_VECREDUCE_AND, G_VECREDUCE_XOR})
       // Try to break down into smaller vectors as long as they're at least 64

llvm/test/CodeGen/AArch64/GlobalISel/legalizer-info-validation.mir

@@ -754,8 +754,8 @@
 # DEBUG-NEXT: .. type index coverage check SKIPPED: user-defined predicate detected
 # DEBUG-NEXT: .. imm index coverage check SKIPPED: user-defined predicate detected
 # DEBUG-NEXT: G_VECREDUCE_MUL (opcode {{[0-9]+}}): 2 type indices, 0 imm indices
-# DEBUG-NEXT: .. type index coverage check SKIPPED: no rules defined
-# DEBUG-NEXT: .. imm index coverage check SKIPPED: no rules defined
+# DEBUG-NEXT: .. type index coverage check SKIPPED: user-defined predicate detected
+# DEBUG-NEXT: .. imm index coverage check SKIPPED: user-defined predicate detected
 # DEBUG-NEXT: G_VECREDUCE_AND (opcode {{[0-9]+}}): 2 type indices, 0 imm indices
 # DEBUG-NEXT: .. opcode {{[0-9]+}} is aliased to {{[0-9]+}}
 # DEBUG-NEXT: .. type index coverage check SKIPPED: user-defined predicate detected