Use full move for byte registers stores in jumps. (#11570)

* Use the 4-byte move for jump spilling.
The other types of spilling already use the proper move:
1) for lsra spilling it always use 4-byte move because we allocate 4-byte slots;
2) for other types it sets needsByteReg and lsra chooses correct register;

We do not apply the second approach to fixing this issue because jmp doesn't
have real uses, that can keep this requirement on. Also, it creates more strict
restrictions, that we need.

Author: Sergey Andreenko

src/jit/codegenxarch.cpp

@@ -5406,8 +5406,9 @@ void CodeGen::genJmpMethod(GenTreePtr jmp)
         // assert should hold.
         assert(varDsc->lvRegNum != REG_STK);
 
-        var_types loadType = varDsc->lvaArgType();
-        getEmitter()->emitIns_S_R(ins_Store(loadType), emitTypeSize(loadType), varDsc->lvRegNum, varNum, 0);
+        assert(!varDsc->lvIsStructField || (compiler->lvaTable[varDsc->lvParentLcl].lvFieldCnt == 1));
+        var_types storeType = genActualType(varDsc->lvaArgType()); // We own the memory and can use the full move.
+        getEmitter()->emitIns_S_R(ins_Store(storeType), emitTypeSize(storeType), varDsc->lvRegNum, varNum, 0);
 
         // Update lvRegNum life and GC info to indicate lvRegNum is dead and varDsc stack slot is going live.
         // Note that we cannot modify varDsc->lvRegNum here because another basic block may not be expecting it.

src/jit/emitxarch.cpp

@@ -4821,6 +4821,12 @@ void emitter::emitIns_S_R(instruction ins, emitAttr attr, regNumber ireg, int va
     UNATIVE_OFFSET sz  = emitInsSizeSV(insCodeMR(ins), varx, offs);
     insFormat      fmt = emitInsModeFormat(ins, IF_SRD_RRD);
 
+#ifdef _TARGET_X86_
+    if (attr == EA_1BYTE)
+    {
+        assert(isByteReg(ireg));
+    }
+#endif
     // 16-bit operand instructions will need a prefix
     if (EA_SIZE(attr) == EA_2BYTE)
     {

src/jit/lclvars.cpp

@@ -1833,7 +1833,10 @@ bool Compiler::lvaShouldPromoteStructVar(unsigned lclNum, lvaStructPromotionInfo
 
             // TODO-PERF - Implement struct promotion for incoming multireg structs
             //             Currently it hits assert(lvFieldCnt==1) in lclvar.cpp line 4417
-
+            //             Also the implementation of jmp uses the 4 byte move to store
+            //             byte parameters to the stack, so that if we have a byte field
+            //             with something else occupying the same 4-byte slot, it will
+            //             overwrite other fields.
             if (structPromotionInfo->fieldCnt != 1)
         {
             JITDUMP("Not promoting promotable struct local V%02u, because lvIsParam is true and #fields = "