Address review feedback: fix docs, remove dead code, use helper

- Fix BaseRegister XML doc: describe as stack base kind (0/1/2), not
  a CPU register number
- Use RegMaskToRegisterNumber helper in x86 GetHeader instead of
  hard-coded register numbers
- Remove unused GetStackBaseRegister/GetSizeOfStackParameterArea
  public methods from GcInfoDecoder (callers use GetHeader)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Max Charlamb

src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Abstractions/Contracts/IGCInfo.cs

@@ -41,7 +41,7 @@ public readonly record struct GCInfoHeader(
 /// <param name="IsRegister">True if the slot is a CPU register; false if it is a stack location.</param>
 /// <param name="RegisterNumber">Register number (meaningful only when IsRegister is true).</param>
 /// <param name="SpOffset">Stack offset from the base (meaningful only when IsRegister is false).</param>
-/// <param name="BaseRegister">Stack base register (meaningful only when IsRegister is false).</param>
+/// <param name="BaseRegister">Stack base kind: 0 = CALLER_SP_REL, 1 = SP_REL, 2 = FRAMEREG_REL (meaningful only when IsRegister is false).</param>
 /// <param name="GcFlags">GC slot flags: 0x1 = interior pointer, 0x2 = pinned, 0x4 = untracked.</param>
 /// <param name="BeginOffset">Code offset where the slot becomes live.</param>
 /// <param name="EndOffset">Code offset where the slot becomes dead (exclusive).</param>

src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/GCInfo/GCInfoDecoder.cs

@@ -555,18 +555,6 @@ public uint GetCodeLength()
         return _codeLength;
     }
 
-    public uint GetStackBaseRegister()
-    {
-        EnsureDecodedTo(DecodePoints.ReversePInvoke);
-        return _stackBaseRegister;
-    }
-
-    public uint GetSizeOfStackParameterArea()
-    {
-        EnsureDecodedTo(DecodePoints.ReversePInvoke);
-        return _fixedStackParameterScratchArea;
-    }
-
     public IReadOnlyList<InterruptibleRange> GetInterruptibleRanges()
     {
         EnsureDecodedTo(DecodePoints.InterruptibleRanges);

src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/GCInfo/X86/GCInfo.cs

@@ -821,7 +821,7 @@ GCInfoHeader IGCInfoDecoder.GetHeader()
             Version: _gcInfoVersion,
             CodeSize: MethodSize,
             PrologSize: Header.PrologSize,
-            StackBaseRegister: (Header.EbpFrame || Header.DoubleAlign) ? 5u : 4u, // EBP=5, ESP=4
+            StackBaseRegister: RegMaskToRegisterNumber((Header.EbpFrame || Header.DoubleAlign) ? RegMask.EBP : RegMask.ESP),
             SizeOfStackParameterArea: 0, // x86 doesn't encode an outgoing scratch area
             IsVarArg: Header.VarArgs,
             WantsReportOnlyLeaf: true,