JIT: Optimize linear mapping of values inside switch

[BoyBaykiller]

https://godbolt.org/z/E1j3nM6c6

Something like the following can frequently happen when converting between enums:

static int GetFuncArgumentReg(int argumentId)
{
    if (argumentId == 0)
    {
        return 4;
    }
    if (argumentId == 1)
    {
        return 5;
    }
    if (argumentId == 2)
    {
        return 6;
    }
    if (argumentId == 3)
    {
        return 7;
    }

    throw new Exception();
}

There is a simple linear mapping: argumentId + 4.

We already have an optimization for GT_SWITCH that turns it into a rangecheck if possible. So adding this would be nothing new. However I see that the above code doesn't get switch-recognized, so that'd be the first thing to delve into probably.

Current assembly:

Program:GetFuncArgumentReg(int):int (FullOpts):
       push     rbp
       push     rbx
       push     rax
       lea      rbp, [rsp+0x10]
       test     edi, edi
       je       SHORT G_M29572_IG09
       cmp      edi, 1
       je       SHORT G_M29572_IG07
       cmp      edi, 2
       je       SHORT G_M29572_IG05
       cmp      edi, 3
       jne      SHORT G_M29572_IG11
       mov      eax, 7
       add      rsp, 8
       pop      rbx
       pop      rbp
       ret      
G_M29572_IG05:  ;; offset=0x0027
       mov      eax, 6
       add      rsp, 8
       pop      rbx
       pop      rbp
       ret      
G_M29572_IG07:  ;; offset=0x0033
       mov      eax, 5
       add      rsp, 8
       pop      rbx
       pop      rbp
       ret      
G_M29572_IG09:  ;; offset=0x003F
       mov      eax, 4
       add      rsp, 8
       pop      rbx
       pop      rbp
       ret      
G_M29572_IG11:  ;; offset=0x004B
       mov      rdi, 0x7D7F3F541690      ; System.Exception
       call     CORINFO_HELP_NEWSFAST
       mov      rbx, rax
       mov      rdi, rbx
       call     [System.Exception:.ctor():this]
       mov      rdi, rbx
       call     CORINFO_HELP_THROW
       int3     

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