[msan][test] Add tests for target("aarch64.svcount") (llvm#164315)

target("aarch64.svcount") is not properly supported by MSan, and will
lead to a crash:
```
fatal error: error in backend: Cannot implicitly convert a scalable size to a fixed-width size in `TypeSize::operator ScalarTy()`
```

This commit adds two test cases: a full test case for tracking any
future improvements to the instrumentation (and also showing the crash),
and a manually reduced test case to show the crash.

Forked from llvm/test/CodeGen/AArch64/sme-aarch64-svcount.ll

Author: thurstond

llvm/test/Instrumentation/MemorySanitizer/AArch64/sme-aarch64-svcount-mini.ll

@@ -0,0 +1,14 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -S -passes=msan -mattr=+sme -o - %s | FileCheck %s
+
+; XFAIL: *
+
+; Forked from llvm/test/CodeGen/AArch64/sme-aarch64-svcount.ll
+; Manually minimized to show MSan leads to a compiler crash
+
+target datalayout = "e-m:e-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128"
+target triple = "aarch64--linux-android9001"
+
+define target("aarch64.svcount") @test_return_arg1(target("aarch64.svcount") %arg0, target("aarch64.svcount") %arg1) nounwind {
+  ret target("aarch64.svcount") %arg1
+}

llvm/test/Instrumentation/MemorySanitizer/AArch64/sme-aarch64-svcount.ll

@@ -0,0 +1,68 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -S -passes=msan -mattr=+sme -o - %s | FileCheck %s
+
+; XFAIL: *
+
+; Forked from llvm/test/CodeGen/AArch64/sme-aarch64-svcount.ll
+
+target datalayout = "e-m:e-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128"
+target triple = "aarch64--linux-android9001"
+
+;
+; Test simple loads, stores and return.
+;
+define target("aarch64.svcount") @test_load(ptr %ptr) nounwind {
+  %res = load target("aarch64.svcount"), ptr %ptr
+  ret target("aarch64.svcount") %res
+}
+
+define void @test_store(ptr %ptr, target("aarch64.svcount") %val) nounwind {
+  store target("aarch64.svcount") %val, ptr %ptr
+  ret void
+}
+
+define target("aarch64.svcount") @test_alloca_store_reload(target("aarch64.svcount") %val) nounwind {
+  %ptr = alloca target("aarch64.svcount"), align 1
+  store target("aarch64.svcount") %val, ptr %ptr
+  %res = load target("aarch64.svcount"), ptr %ptr
+  ret target("aarch64.svcount") %res
+}
+
+;
+; Test passing as arguments (from perspective of callee)
+;
+
+define target("aarch64.svcount") @test_return_arg1(target("aarch64.svcount") %arg0, target("aarch64.svcount") %arg1) nounwind {
+  ret target("aarch64.svcount") %arg1
+}
+
+define target("aarch64.svcount") @test_return_arg4(target("aarch64.svcount") %arg0, target("aarch64.svcount") %arg1, target("aarch64.svcount") %arg2, target("aarch64.svcount") %arg3, target("aarch64.svcount") %arg4) nounwind {
+  ret target("aarch64.svcount") %arg4
+}
+
+;
+; Test passing as arguments (from perspective of caller)
+;
+
+declare void @take_svcount_1(target("aarch64.svcount") %arg)
+define void @test_pass_1arg(target("aarch64.svcount") %arg) nounwind {
+  call void @take_svcount_1(target("aarch64.svcount") %arg)
+  ret void
+}
+
+declare void @take_svcount_5(target("aarch64.svcount") %arg0, target("aarch64.svcount") %arg1, target("aarch64.svcount") %arg2, target("aarch64.svcount") %arg3, target("aarch64.svcount") %arg4)
+define void @test_pass_5args(target("aarch64.svcount") %arg) nounwind {
+  call void @take_svcount_5(target("aarch64.svcount") %arg, target("aarch64.svcount") %arg, target("aarch64.svcount") %arg, target("aarch64.svcount") %arg, target("aarch64.svcount") %arg)
+  ret void
+}
+
+define target("aarch64.svcount") @test_sel(target("aarch64.svcount") %x, target("aarch64.svcount") %y, i1 %cmp) sanitize_memory {
+  %x.y = select i1 %cmp, target("aarch64.svcount") %x, target("aarch64.svcount") %y
+  ret target("aarch64.svcount") %x.y
+}
+
+define target("aarch64.svcount") @test_sel_cc(target("aarch64.svcount") %x, target("aarch64.svcount") %y, i32 %k) sanitize_memory {
+  %cmp = icmp sgt i32 %k, 42
+  %x.y = select i1 %cmp, target("aarch64.svcount") %x, target("aarch64.svcount") %y
+  ret target("aarch64.svcount") %x.y
+}