Improve convert_hex_to_binary x86_64 codegen.

[ttsugriy]

This results in shorter and branchless x86_64 assembly: https://godbolt.org/z/5jqTbYWWh

Original:

convert_hex_to_binary(char):             # @convert_hex_to_binary(char)
        mov     eax, edi
        cmp     al, 57
        jg      .LBB0_2
        add     eax, -48
        ret
.LBB0_2:
        xor     ecx, ecx
        cmp     al, 97
        setb    cl
        shl     ecx, 5
        add     eax, ecx
        add     eax, -87
        ret

and new version:

convert_hex_to_binary(char):             # @convert_hex_to_binary(char)
        xor     ecx, ecx
        cmp     dil, 97
        setl    cl
        shl     ecx, 5
        add     ecx, -87
        cmp     dil, 58
        mov     eax, -48
        cmovge  eax, ecx
        add     eax, edi
        ret