Refactor memory copies in SafeBuffer

[xtqqczze]

No diffs expected, except from the changes to sign-extension.

Diffs are an improvement: MihuBot/runtime-utils#548

System.Runtime.InteropServices.SafeBuffer:Read[ubyte](ulong):ubyte:this

 G_M48575_IG03:
-       lea      rsi, [rbp-0x20]
-       mov      rdi, gword ptr [rbp-0x28]
+       lea      rsi, [rbp-0x10]
+       mov      rdi, gword ptr [rbp-0x18]
        mov      rax, 0xD1FFAB1E      ; code for System.Runtime.InteropServices.SafeHandle:DangerousAddRef(byref):this
        call     [rax]System.Runtime.InteropServices.SafeHandle:DangerousAddRef(byref):this
-       mov      edx, r15d
-       lea      rdi, [rbp-0x18]
-       mov      rsi, rbx
-       mov      rax, 0xD1FFAB1E      ; code for System.SpanHelpers:Memmove(byref,byref,ulong)
-       call     [rax]System.SpanHelpers:Memmove(byref,byref,ulong)
-       nop      
-						;; size=43 bbWeight=1 PerfScore 9.25
+       movzx    rbx, byte  ptr [rbx]
+						;; size=23 bbWeight=1 PerfScore 6.75

[xtqqczze]

@MihuBot -arm64

[xtqqczze]

@MihuBot -arm64

[xtqqczze]

@MihuBot

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[agocke]

@tannergooding Do you want to take a look at this?