InstSimplify: stripAndComputeConstantOffsets can be called with vectors of pointers too.

Prepare it for vectors of pointers and handle simple cases. We don't handle
complicated cases because accumulateConstantOffset bails on pointer vectors.
Fixes selfhost on i386.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174179 91177308-0d34-0410-b5e6-96231b3b80d8

test/Transforms/InstSimplify/ptr_diff.ll

@@ -46,3 +46,14 @@ define i64 @ptrdiff3(i8* %ptr) {
   %diff = sub i64 %last.int, %first.int
   ret i64 %diff
 }
+
+define <4 x i32> @ptrdiff4(<4 x i8*> %arg) nounwind {
+; Handle simple cases of vectors of pointers.
+; CHECK: @ptrdiff4
+; CHECK: ret <4 x i32> zeroinitializer
+  %p1 = ptrtoint <4 x i8*> %arg to <4 x i32>
+  %bc = bitcast <4 x i8*> %arg to <4 x i32*>
+  %p2 = ptrtoint <4 x i32*> %bc to <4 x i32>
+  %sub = sub <4 x i32> %p1, %p2
+  ret <4 x i32> %sub
+}