JIT: Merge store/loads for return buffers

[EgorBo]

A follow up to #112060

Previously, we were conservative about return buffers and couldn't really merge loads/store due to unknown alignment & unknown atomicity requirements.

(int,int,int,int) Test => (1, 2, 3, 4);

; Method Proga:get_Test():System.ValueTuple`4[int,int,int,int]:this
-      mov      dword ptr [rdx], 1
-      mov      dword ptr [rdx+0x04], 2
-      mov      dword ptr [rdx+0x08], 3
-      mov      dword ptr [rdx+0x0C], 4
+      vmovups  xmm0, xmmword ptr [reloc @RWD00]
+      vmovups  xmmword ptr [rdx], xmm0
       mov      rax, rdx
       ret   
+RWD00  	dq	0000000200000001h, 0000000400000003h   
+; Total bytes of code: 31
-; Total bytes of code: 16

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[EgorBo]

PTAL @jakobbotsch @dotnet/jit-contrib diffs aren't too big, although, we don't have 4 SPMI collections