[LoongArch] Return true from shouldConsiderGEPOffsetSplit

If not performing gep splits can prevent important optimizations, such
as preventing the element indices / member offsets from being
(partially) folded into load/store instruction immediates.

Author: wangleiat

llvm/lib/Target/LoongArch/LoongArchISelLowering.h

@@ -238,6 +238,7 @@ class LoongArchTargetLowering : public TargetLowering {
   bool isShuffleMaskLegal(ArrayRef<int> Mask, EVT VT) const override {
     return false;
   }
+  bool shouldConsiderGEPOffsetSplit() const override { return true; }
 
 private:
   /// Target-specific function used to lower LoongArch calling conventions.

llvm/test/Transforms/CodeGenPrepare/LoongArch/lit.local.cfg

@@ -0,0 +1,2 @@
+if not "LoongArch" in config.root.targets:
+    config.unsupported = True

llvm/test/Transforms/CodeGenPrepare/LoongArch/splitgep.ll

@@ -0,0 +1,34 @@
+; RUN: opt --mtriple=loongarch64 -S --passes='require<profile-summary>,function(codegenprepare)' %s | FileCheck %s
+
+; Check that we have deterministic output
+define void @test(ptr %sp, ptr %t, i32 %n) {
+; CHECK-LABEL: @test(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    %splitgep1 = getelementptr i8, ptr %t, i64 80000
+; CHECK-NEXT:    %s = load ptr, ptr %sp
+; CHECK-NEXT:    %splitgep = getelementptr i8, ptr %s, i64 80000
+entry:
+  %s = load ptr, ptr %sp
+  br label %while_cond
+
+while_cond:
+  %phi = phi i32 [ 0, %entry ], [ %i, %while_body ]
+  %gep0 = getelementptr [65536 x i32], ptr %s, i64 0, i64 20000
+  %gep1 = getelementptr [65536 x i32], ptr %s, i64 0, i64 20001
+  %gep2 = getelementptr [65536 x i32], ptr %t, i64 0, i64 20000
+  %gep3 = getelementptr [65536 x i32], ptr %t, i64 0, i64 20001
+  %cmp = icmp slt i32 %phi, %n
+  br i1 %cmp, label %while_body, label %while_end
+
+while_body:
+  %i = add i32 %phi, 1
+  store i32 %i, ptr %gep0
+  store i32 %phi, ptr %gep1
+  store i32 %i, ptr %gep2
+  store i32 %phi, ptr %gep3
+  br label %while_cond
+
+while_end:
+  ret void
+}
+