[RyuJit Wasm] don't home locals if none are referenced

[AndyAyersMS]

In such cases we won't have set up a stack pointer.

[Copilot]

Pull request overview

This PR updates the WASM JIT codegen to avoid homing parameters when no locals or arguments require stack-based storage, i.e., when no stack pointer register has been set up.

Changes:

• Add a guard in CodeGen::genHomeRegisterParams that checks GetStackPointerReg() and skips parameter homing when it is REG_NA.
• Assert that the frame pointer is not used in this frameless case and emit a JITDUMP message for debugging.

[AndyAyersMS]

@dotnet/jit-contrib PTAL
@SingleAccretion let me know if you think we should handle this some other way.

Fixes codegen for methods with no (user) arguments, like

    public static int Zero() => 0;

[dotnet-policy-service]

Tagging subscribers to 'arch-wasm': @lewing, @pavelsavara
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[SingleAccretion]

I think this is a situation where the RA is wrong; with implicitly referenced variables we have to assume there may be some references to them outside of IR. So something like below:

diff --git a/src/coreclr/jit/regallocwasm.cpp b/src/coreclr/jit/regallocwasm.cpp
index 48bcd978abd..8c001fa77e7 100644
--- a/src/coreclr/jit/regallocwasm.cpp
+++ b/src/coreclr/jit/regallocwasm.cpp
@@ -100,6 +100,10 @@ void WasmRegAlloc::IdentifyCandidates()
         {
             JITDUMP("RA candidate: V%02u", lclNum);
         }
+        else if ((m_fpReg == REG_NA) && (varDsc->lvRefCnt() != 0))
+        {
+            m_fpReg = AllocateFreeRegister(TYP_I_IMPL);
+        }
     }
 }
 
@@ -147,11 +151,6 @@ void WasmRegAlloc::AllocateAndResolveNode(GenTree* node)
             {
                 RewriteLocalStackStore(node->AsLclVarCommon());
             }
-
-            if (m_fpReg == REG_NA)
-            {
-                m_fpReg = AllocateFreeRegister(TYP_I_IMPL);
-            }
         }
     }
     else if (node->OperIs(GT_LCLHEAP))
diff --git a/src/coreclr/jit/registeropswasm.cpp b/src/coreclr/jit/registeropswasm.cpp
index 0386d477d1b..b8f96cc35a7 100644
--- a/src/coreclr/jit/registeropswasm.cpp
+++ b/src/coreclr/jit/registeropswasm.cpp
@@ -109,7 +109,10 @@ unsigned UnpackWasmReg(regNumber reg, WasmValueType* pType)
 //
 unsigned WasmRegToIndex(regNumber reg)
 {
-    return UnpackWasmReg(reg);
+    WasmValueType wasmType;
+    unsigned index = UnpackWasmReg(reg, &wasmType);
+    assert((WasmValueType::Invalid < wasmType) && (wasmType < WasmValueType::Count));
+    return index;
 }
 
 bool genIsValidReg(regNumber reg)