[llvm] [GlobalISel] support lowering of G_SHUFFLEVECTOR with pointer args (PR #141959)

[Matt Arsenault via llvm-commits]

================
@@ -0,0 +1,21 @@
+# RUN: llc -march=amdgcn -run-pass=legalizer -verify-machineinstrs %s -o - 
+
+--- |
+  define i32 @test(i64 %arg) {
+    ret i32 0
+  }
+...
+name:            test
+tracksRegLiveness: true
+body: |
+  bb.0:
+    %9:_(p0) = G_CONSTANT i64 0
+    %10:_(<2 x p0>) = G_BUILD_VECTOR %9:_(p0), %9:_(p0)
+    %8:_(<2 x p0>) = G_LOAD %9:_(p0) :: (load (<2 x p0>) from `ptr null`, align 64)
----------------
arsenm wrote:

```suggestion
    %8:_(<2 x p0>) = G_LOAD %9:_(p0) :: (load (<2 x p0>))
```

https://github.com/llvm/llvm-project/pull/141959