Make structs readonly when applicable (#4002)

* Make all structs readonly when applicable. It should reduce amount of needless defensive copies

* Make structs with trivial boilerplate equality code record structs

* Remove unnecessary readonly modifiers from TextureCreateInfo

* Make BitMap structs readonly too

ARMeilleure/CodeGen/RegisterAllocators/AllocationResult.cs

@@ -1,6 +1,6 @@
 namespace ARMeilleure.CodeGen.RegisterAllocators
 {
-    struct AllocationResult
+    readonly struct AllocationResult
     {
         public int IntUsedRegisters { get; }
         public int VecUsedRegisters { get; }

ARMeilleure/CodeGen/RegisterAllocators/CopyResolver.cs

@@ -11,7 +11,7 @@ namespace ARMeilleure.CodeGen.RegisterAllocators
     {
         private class ParallelCopy
         {
-            private struct Copy
+            private readonly struct Copy
             {
                 public Register Dest   { get; }
                 public Register Source { get; }

ARMeilleure/CodeGen/RegisterAllocators/HybridAllocator.cs

@@ -11,7 +11,7 @@ namespace ARMeilleure.CodeGen.RegisterAllocators
 {
     class HybridAllocator : IRegisterAllocator
     {
-        private struct BlockInfo
+        private readonly struct BlockInfo
         {
             public bool HasCall { get; }
 

ARMeilleure/CodeGen/RegisterAllocators/RegisterMasks.cs

@@ -3,7 +3,7 @@ using System;
 
 namespace ARMeilleure.CodeGen.RegisterAllocators
 {
-    struct RegisterMasks
+    readonly struct RegisterMasks
     {
         public int IntAvailableRegisters   { get; }
         public int VecAvailableRegisters   { get; }

ARMeilleure/CodeGen/X86/IntrinsicInfo.cs

@@ -1,6 +1,6 @@
 namespace ARMeilleure.CodeGen.X86
 {
-    struct IntrinsicInfo
+    readonly struct IntrinsicInfo
     {
         public X86Instruction Inst { get; }
         public IntrinsicType  Type { get; }