[AArch64AsmParser] Support (xxx), lsl 16 after #80571

An immediate integer operand not prefixed with # can also be followed
by "lsl". Parse "lsl".
In the wild, edk2 ArmPkg/Include/AsmMacroIoLibV8.h has
`movz      Reg, (Val) >> 16, lsl #16`.

Note: our support for paren expression not prefixed with # is not very
good. For example, `adds x3, x4, (1024>>0), lsl 12` fails to be parsed.