[AArch64][NFC] Add test as a representative of scalarizing a vector integer division

The last resort to vectorize a bundle of integer divisions is considered scalarizing it.
Currently, the cost estimates for scalarizing a vector division can be considerably overestimated
as is the scenario with this motivating test case i.e. vector cost should not deviate much from
the  scalar cost.

Future patch will try to improve the scalarization cost.

Author: sushgokh

llvm/test/Transforms/SLPVectorizer/AArch64/scalarizing-vector-cost.ll

@@ -0,0 +1,23 @@
+; RUN: opt -mtriple=aarch64 -passes=slp-vectorizer -debug-only=SLP -S -disable-output < %s 2>&1 | FileCheck %s
+
+define <4 x i8> @v4i8(<4 x i8> %a, <4 x i8> %b)
+{
+; CHECK: SLP: Found cost = 18 for VF=4
+  %a0 = extractelement <4 x i8> %a, i64 0
+  %a1 = extractelement <4 x i8> %a, i64 1
+  %a2 = extractelement <4 x i8> %a, i64 2
+  %a3 = extractelement <4 x i8> %a, i64 3
+  %b0 = extractelement <4 x i8> %b, i64 0
+  %b1 = extractelement <4 x i8> %b, i64 1
+  %b2 = extractelement <4 x i8> %b, i64 2
+  %b3 = extractelement <4 x i8> %b, i64 3
+  %1 = sdiv i8 %a0, undef
+  %2 = sdiv i8 %a1, 1
+  %3 = sdiv i8 %a2, 2
+  %4 = sdiv i8 %a3, 4
+  %r0 = insertelement <4 x i8> poison, i8 %1, i32 0
+  %r1 = insertelement <4 x i8> %r0, i8 %2, i32 1
+  %r2 = insertelement <4 x i8> %r1, i8 %3, i32 2
+  %r3 = insertelement <4 x i8> %r2, i8 %4, i32 3
+  ret <4 x i8> %r3
+}