Rewrite the parsing of register expressions. GCC, for reasons unknown, put the displacement in the middle.

I am completely restructuring the parser definition so that they are more explicit.  They are more verbose too, but at least I understand what they do.

tests/test_files/gs_x86_gcc.s

@@ -0,0 +1,102 @@
+# Produced with gcc 14.2 with -O3 -march=sapphirerapids -fopenmp-simd -mprefer-vector-width=512, https://godbolt.org/z/drE47x1b4.
+.LC3:
+        .string "%f\n"
+main:
+        push    r14
+        xor     edi, edi
+        push    r13
+        push    r12
+        push    rbp
+        push    rbx
+        call    time
+        mov     edi, eax
+        call    srand
+        mov     edi, 1600
+        call    malloc
+        mov     r12, rax
+        mov     rbp, rax
+        lea     r13, [rax+1600]
+        mov     rbx, rax
+.L2:
+        mov     edi, 1600
+        add     rbx, 8
+        call    malloc
+        mov     QWORD PTR [rbx-8], rax
+        cmp     r13, rbx
+        jne     .L2
+        lea     rbx, [r12+8]
+        lea     r13, [r12+1592]
+.L5:
+        mov     r14d, 8
+.L4:
+        call    rand
+        vxorpd  xmm2, xmm2, xmm2
+        mov     rcx, QWORD PTR [rbx]
+        movsx   rdx, eax
+        mov     esi, eax
+        imul    rdx, rdx, 351843721
+        sar     esi, 31
+        sar     rdx, 45
+        sub     edx, esi
+        imul    edx, edx, 100000
+        sub     eax, edx
+        vcvtsi2sd       xmm0, xmm2, eax
+        vdivsd  xmm0, xmm0, QWORD PTR .LC0[rip]
+        vmovsd  QWORD PTR [rcx+r14], xmm0
+        add     r14, 8
+        cmp     r14, 1592
+        jne     .L4
+        add     rbx, 8
+        cmp     r13, rbx
+        jne     .L5
+        vmovsd  xmm1, QWORD PTR .LC1[rip]
+        lea     rdi, [r12+1584]
+.L6:
+        mov     rdx, QWORD PTR [rbp+8]
+        mov     rcx, QWORD PTR [rbp+16]
+        mov     eax, 1
+        mov     rsi, QWORD PTR [rbp+0]
+        vmovsd  xmm0, QWORD PTR [rdx]
+.L7:
+        vaddsd  xmm0, xmm0, QWORD PTR [rcx+rax*8]
+        vaddsd  xmm0, xmm0, QWORD PTR [rdx+8+rax*8]
+        vaddsd  xmm0, xmm0, QWORD PTR [rsi+rax*8]
+        vmulsd  xmm0, xmm0, xmm1
+        vmovsd  QWORD PTR [rdx+rax*8], xmm0
+        inc     rax
+        cmp     rax, 199
+        jne     .L7
+        vmovsd  xmm0, QWORD PTR [rdx+1592]
+        add     rbp, 8
+        vmovsd  QWORD PTR [rcx+8], xmm0
+        cmp     rdi, rbp
+        jne     .L6
+        mov     rax, QWORD PTR [r12+1584]
+        vmovsd  xmm0, QWORD PTR .LC2[rip]
+        vucomisd        xmm0, QWORD PTR [rax+1584]
+        jp      .L9
+        je      .L19
+.L9:
+        pop     rbx
+        xor     eax, eax
+        pop     rbp
+        pop     r12
+        pop     r13
+        pop     r14
+        ret
+.L19:
+        mov     rax, QWORD PTR [r12]
+        mov     edi, OFFSET FLAT:.LC3
+        vmovsd  xmm0, QWORD PTR [rax]
+        mov     eax, 1
+        call    printf
+        jmp     .L9
+.LC0:
+        .long   0
+        .long   1083129856
+.LC1:
+        .long   2061584302
+        .long   1072934420
+.LC2:
+        .long   -57724360
+        .long   1072939201