[AArch64AsmParser] Allow branch target symbol to have a shift/extend modifier name (#80571)

Shift and extend modifiers are parsed as separate operands. When a
symbol operand of a branch instruction has such a "bad" name,
AArch64AsmParser will report an error.

```
% cat a.c
void lsl(); void lsr(); void asr(); void ror(); void uxtb(); void sxtx();
void foo() { lsl(); asr(); asr(); ror(); uxtb(); sxtx(); }
% clang --target=aarch64 -c -save-temps a.c
a.s:15:8: error: expected #imm after shift specifier
        bl      lsl
                   ^
a.s:16:8: error: expected #imm after shift specifier
        bl      asr
                   ^
a.s:17:8: error: expected #imm after shift specifier
        bl      asr
                   ^
a.s:18:8: error: expected #imm after shift specifier
        bl      ror
                   ^
a.s:19:5: error: expected label or encodable integer pc offset
        bl      uxtb
                ^
a.s:20:5: error: expected label or encodable integer pc offset
        bl      sxtx
                ^
```

In contrast, gas correctly parses these instructions.

Fix #79729 by parsing shift/extend modifier after an immediate
value/register