JIT: unroll more memset patterns

[EgorBo]

Currently, JIT always gives up on unrolling Memset(Fill) for anything other than byte or zero. This PR enables it for all non-zero primitives.

void Test(Span<char> span) => span.Slice(0, 10).Fill('x');

Current codegen:

; Method Program:Test(System.Span`1[ushort]):this (FullOpts)
G_M11560_IG01:
       sub      rsp, 40
G_M11560_IG02:
       cmp      dword ptr [rdx+0x08], 10
       jl       SHORT G_M11560_IG04
       mov      rcx, bword ptr [rdx]
       mov      edx, 10
       mov      r8d, 120
       call     [System.SpanHelpers:Fill[ushort](byref,ulong,ushort)]
       nop      
G_M11560_IG03:
       add      rsp, 40
       ret      
G_M11560_IG04:
       call     [System.ThrowHelper:ThrowArgumentOutOfRangeException()]
       int3     
; Total bytes of code: 43

New codegen:

; Method Program:Test(System.Span`1[ushort]):this (FullOpts)
G_M11560_IG01:
       sub      rsp, 40
G_M11560_IG02:
       cmp      dword ptr [rdx+0x08], 10
       jl       SHORT G_M11560_IG04
       mov      rax, bword ptr [rdx]
       vmovups  xmm0, xmmword ptr [reloc @RWD00]
       vmovups  xmmword ptr [rax], xmm0
       mov      dword ptr [rax+0x10], 0x780078
G_M11560_IG03:
       add      rsp, 40
       ret      
G_M11560_IG04:
       call     [System.ThrowHelper:ThrowArgumentOutOfRangeException()]
       int3     
RWD00  	dq	0078007800780078h, 0078007800780078h
; Total bytes of code: 44

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