[AArch64] Make apple-m4 armv8.7-a again (from armv9.2-a).

This is a partial revert of c66e1d6.  Even though that
allowed us to declare v9.2-a support without picking up SVE2
in both the backend and the driver, the frontend itself still
enabled SVE via the arch version's default extensions.

Avoid that by reverting back to v8.7-a while we look into
longer-term solutions.

Author: ahmedbougacha

clang/test/CodeGen/aarch64-targetattr.c

@@ -218,7 +218,7 @@ void applem4() {}
 // CHECK: attributes #[[ATTR15]] = { noinline nounwind optnone "branch-target-enforcement" "guarded-control-stack" "no-trapping-math"="true" "sign-return-address"="non-leaf" "sign-return-address-key"="a_key" "stack-protector-buffer-size"="8" "target-cpu"="neoverse-n1" "target-features"="+aes,+bf16,+bti,+ccidx,+complxnum,+crc,+dit,+dotprod,+flagm,+fp-armv8,+fullfp16,+i8mm,+jsconv,+lse,+neon,+pauth,+perfmon,+predres,+ras,+rcpc,+rdm,+sb,+sha2,+spe,+ssbs,+sve,+sve2,+v8.1a,+v8.2a,+v8.3a,+v8.4a,+v8.5a,+v8.6a,+v8a" "tune-cpu"="cortex-a710" }
 // CHECK: attributes #[[ATTR16]] = { noinline nounwind optnone "no-trapping-math"="true" "stack-protector-buffer-size"="8" }
 // CHECK: attributes #[[ATTR17]] = { noinline nounwind optnone "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-features"="-v9.3a" }
-// CHECK: attributes #[[ATTR18]] = { noinline nounwind optnone "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="apple-m4" "target-features"="+aes,+bf16,+bti,+ccidx,+complxnum,+crc,+dit,+dotprod,+flagm,+fp-armv8,+fp16fml,+fpac,+fullfp16,+i8mm,+jsconv,+lse,+mec,+neon,+pauth,+perfmon,+predres,+ras,+rcpc,+rdm,+rme,+sb,+sha2,+sha3,+sme,+sme-f64f64,+sme-i16i64,+sme2,+ssbs,+sve,+sve2,+v8.1a,+v8.2a,+v8.3a,+v8.4a,+v8.5a,+v8.6a,+v8.7a,+v8a,+v9.1a,+v9.2a,+v9a,+wfxt" }
+// CHECK: attributes #[[ATTR18]] = { noinline nounwind optnone "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="apple-m4" "target-features"="+aes,+bf16,+bti,+ccidx,+complxnum,+crc,+dit,+dotprod,+flagm,+fp-armv8,+fp16fml,+fpac,+fullfp16,+i8mm,+jsconv,+lse,+neon,+pauth,+perfmon,+predres,+ras,+rcpc,+rdm,+sb,+sha2,+sha3,+sme,+sme-f64f64,+sme-i16i64,+sme2,+ssbs,+v8.1a,+v8.2a,+v8.3a,+v8.4a,+v8.5a,+v8.6a,+v8.7a,+v8a,+wfxt" }
 //.
 // CHECK: [[META0:![0-9]+]] = !{i32 1, !"wchar_size", i32 4}
 // CHECK: [[META1:![0-9]+]] = !{!"{{.*}}clang version {{.*}}"}

llvm/lib/Target/AArch64/AArch64Processors.td

@@ -895,7 +895,12 @@ def ProcessorFeatures {
                                      FeatureLSE, FeaturePAuth, FeatureFPAC,
                                      FeatureRAS, FeatureRCPC, FeatureRDM,
                                      FeatureBF16, FeatureDotProd, FeatureMatMulInt8, FeatureSSBS];
-  list<SubtargetFeature> AppleM4 = [HasV9_2aOps, FeatureSHA2, FeatureFPARMv8,
+  // Technically apple-m4 is v9.2a, but we can't use that here.
+  // Historically, llvm defined v9.0a as requiring SVE, but it's optional
+  // according to the Arm ARM, and not supported by the core.  We decoupled the
+  // two in the clang driver and in the backend subtarget features, but it's
+  // still an issue in the clang frontend.  v8.7a is the next closest choice.
+  list<SubtargetFeature> AppleM4 = [HasV8_7aOps, FeatureSHA2, FeatureFPARMv8,
                                     FeatureNEON, FeaturePerfMon, FeatureSHA3,
                                     FeatureFullFP16, FeatureFP16FML,
                                     FeatureAES, FeatureBF16,

llvm/unittests/TargetParser/TargetParserTest.cpp

@@ -1122,7 +1122,7 @@ INSTANTIATE_TEST_SUITE_P(
                       AArch64CPUTestParams("apple-a16", "armv8.6-a"),
                       AArch64CPUTestParams("apple-m3", "armv8.6-a"),
                       AArch64CPUTestParams("apple-a17", "armv8.6-a"),
-                      AArch64CPUTestParams("apple-m4", "armv9.2-a"),
+                      AArch64CPUTestParams("apple-m4", "armv8.7-a"),
                       AArch64CPUTestParams("exynos-m3", "armv8-a"),
                       AArch64CPUTestParams("exynos-m4", "armv8.2-a"),
                       AArch64CPUTestParams("exynos-m5", "armv8.2-a"),