Fix failed assertion 'FPbased == FPbased2' (#111787)

* Move assert to prevent incorrect trigger when lvaFrameAddress optimization merged SP+n and FP-m stores into a single pair-wise store

* Better fix

src/coreclr/jit/emitarm64.cpp

@@ -12311,7 +12311,7 @@ size_t emitter::emitOutputInstr(insGroup* ig, instrDesc* id, BYTE** dp)
 
                 // If there are 2 GC vars in this instrDesc, get the 2nd variable
                 // that should be tracked.
-                adr2     = emitComp->lvaFrameAddress(varNum2, &FPbased2, true);
+                adr2     = emitComp->lvaFrameAddress(varNum2, &FPbased2, FPbased);
                 ofs2Dist = EA_SIZE_IN_BYTES(size);
 #ifdef DEBUG
                 assert(FPbased == FPbased2);