Move RBM_SAVED_LOCALLOC_SP reservation from compCompile to setFrameType in LSRA (#121745)

Move the code that adds `RBM_SAVED_LOCALLOC_SP` to `rsMaskResvd` from
`compCompile` into `setFrameType` in LSRA

## Plan
- [x] Understand the problem: Need to move ARM-specific code that
reserves a register for localloc stack unwinding
- [x] Locate source code:
  - Code to move: `compiler.cpp` lines 4953-4959 (inside `compCompile`)
  - Destination: `lsra.cpp` `setFrameType()` function after line 2718
- [x] Verify baseline build (CoreCLR) succeeds
- [x] Make the code change:
  - Removed the code block from `compiler.cpp` (lines 4953-4959)
- Added the code block to `lsra.cpp` in `setFrameType()` after existing
`rsMaskResvd` setting
- [x] Build CoreCLR to verify compilation succeeds
- [x] Address review feedback: Add JITDUMP statement for register
reservation
- [x] Build CoreCLR again to verify JITDUMP addition
- [x] Run CodeQL security scan - No issues found
- [x] Complete

## Changes Made
- **src/coreclr/jit/compiler.cpp**: Removed the `#ifdef TARGET_ARM`
block that sets `RBM_SAVED_LOCALLOC_SP` in `rsMaskResvd` from the
`compCompile` function
- **src/coreclr/jit/lsra.cpp**: Added the same block to `setFrameType()`
after the existing code that sets `rsMaskResvd` for large frame offsets
- **src/coreclr/jit/lsra.cpp**: Added JITDUMP statement to log when
`REG_SAVED_LOCALLOC_SP` is reserved

## Build Status
✅ CoreCLR build succeeded with changes
✅ CoreCLR build succeeded with JITDUMP addition
✅ No CodeQL security issues detected

## Security Summary
No security vulnerabilities were introduced or detected in this change.
The code is a straightforward refactoring that moves existing logic to a
more appropriate location without changing its behavior.

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<details>

<summary>Original prompt</summary>

> Move the code that adds `RBM_SAVED_LOCALLOC_SP` to `rsMaskResvd` from
`compCompile` into `setFrameType` in LSRA, next to the existing code
that sets `rsMaskResvd`


</details>



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Author: Copilot

src/coreclr/jit/compiler.cpp

@@ -4950,14 +4950,6 @@ void Compiler::compCompile(void** methodCodePtr, uint32_t* methodCodeSize, JitFl
     // call and register argument info, flowgraph and loop info, etc.
     compJitStats();
 
-#ifdef TARGET_ARM
-    if (compLocallocUsed)
-    {
-        // We reserve REG_SAVED_LOCALLOC_SP to store SP on entry for stack unwinding
-        codeGen->regSet.rsMaskResvd |= RBM_SAVED_LOCALLOC_SP;
-    }
-#endif // TARGET_ARM
-
     if (compIsAsync())
     {
         DoPhase(this, PHASE_ASYNC, &Compiler::TransformAsync);

src/coreclr/jit/lsra.cpp

@@ -2588,6 +2588,16 @@ void LinearScan::setFrameType()
     }
 #endif // TARGET_ARMARCH || TARGET_RISCV64
 
+#ifdef TARGET_ARM
+    if (compiler->compLocallocUsed)
+    {
+        // We reserve REG_SAVED_LOCALLOC_SP to store SP on entry for stack unwinding
+        compiler->codeGen->regSet.rsMaskResvd |= RBM_SAVED_LOCALLOC_SP;
+        JITDUMP("  Reserved REG_SAVED_LOCALLOC_SP (%s) due to localloc\n", getRegName(REG_SAVED_LOCALLOC_SP));
+        removeMask |= RBM_SAVED_LOCALLOC_SP.GetIntRegSet();
+    }
+#endif // TARGET_ARM
+
     if ((removeMask != RBM_NONE) && ((availableIntRegs & removeMask) != 0))
     {
         // We know that we're already in "read mode" for availableIntRegs. However,