[LV] Add pre-commit crash test for widengep-narrow-scalable

Reduced test case from llvm-test-suite, reported by David Spickett.

Author: artagnon

llvm/test/Transforms/LoopVectorize/narrow-to-single-scalar-widen-gep-scalable.ll

@@ -0,0 +1,20 @@
+; RUN: not --crash opt -p loop-vectorize -force-vector-width=2 \
+; RUN:  -force-target-supports-scalable-vectors=true \
+; RUN:  -scalable-vectorization=preferred -S %s
+
+define void @widengep_narrow(ptr %in, ptr noalias %p) {
+entry:
+  br label %loop
+
+loop:
+  %iv = phi i64 [ 0, %entry ], [ %iv.next, %loop ]
+  %gep.in.off = getelementptr i8, ptr %in, i64 8
+  %gep.in.iv = getelementptr i32, ptr %gep.in.off, i64 %iv
+  store ptr %gep.in.iv, ptr %p
+  %iv.next = add i64 %iv, 1
+  %ec = icmp eq i64 %iv, 1024
+  br i1 %ec, label %exit, label %loop
+
+exit:
+  ret void
+}