[LoongArch] Return true from shouldConsiderGEPOffsetSplit (#88371)

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/splitgep.ll

@@ -4,9 +4,9 @@
 define void @test(ptr %sp, ptr %t, i32 %n) {
 ; CHECK-LABEL: @test(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    %s = load ptr, ptr %sp, align 8
-; CHECK-NEXT:    br label %while_cond
-
+; 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