[AArch64] Make wfxt a full Extension (#90987)

Before #90987 WFxT did not have an AEK_WFXT, so it was assumed to be an
FMV-only extension. However it also had a SubtargetFeature. This commit
combines the two. This fixes an issue where -mattr=+wfxt was ignored,
but has the side effect of allowing +wfxt as an option to -march.

llvm/lib/Target/AArch64/AArch64Features.td

@@ -90,7 +90,6 @@ def : FMVOnlyExtension<"FEAT_SVE_BF16", "sve-bf16", "+sve,+bf16,+fullfp16,+fp-ar
 def : FMVOnlyExtension<"FEAT_SVE_EBF16", "sve-ebf16", "+sve,+bf16,+fullfp16,+fp-armv8,+neon", 330>;
 def : FMVOnlyExtension<"FEAT_SVE_I8MM", "sve-i8mm", "+sve,+i8mm,+fullfp16,+fp-armv8,+neon", 340>;
 def : FMVOnlyExtension<"FEAT_SVE_PMULL128", "sve2-pmull128", "+sve2,+sve,+sve2-aes,+fullfp16,+fp-armv8,+neon", 390>;
-def : FMVOnlyExtension<"FEAT_WFXT", "wfxt", "+wfxt", 550>;
 
 
 // Each SubtargetFeature which corresponds to an Arm Architecture feature should
@@ -596,9 +595,9 @@ def FeatureMatMulFP64 : Extension<"f64mm", "MatMulFP64",
 def FeatureXS : SubtargetFeature<"xs", "HasXS",
     "true", "Enable Armv8.7-A limited-TLB-maintenance instruction (FEAT_XS)">;
 
-// FIXME link with FMVExtension?
-def FeatureWFxT : SubtargetFeature<"wfxt", "HasWFxT",
-    "true", "Enable Armv8.7-A WFET and WFIT instruction (FEAT_WFxT)">;
+def FeatureWFxT : Extension<"wfxt", "WFxT",
+    "Enable Armv8.7-A WFET and WFIT instruction (FEAT_WFxT)", [],
+    "FEAT_WFXT", "", 550>;
 
 def FeatureHCX : SubtargetFeature<
     "hcx", "HasHCX", "true", "Enable Armv8.7-A HCRX_EL2 system register (FEAT_HCX)">;