arm64/disassem.c: remove redundant OP_RN_SP for TYPE_02

[koliagogsadze_gmail.com]

Summary

Removed redundant OP_RN_SP for TYPE_02, since addressing modes use Xn or SP register.
Added helper function to get Xn or SP.

[mhorne]

sys/arm64/arm64/disassem.c

101–105

Where does this naming/formatting come from, compared to the existing stuff? I guess from the ARM documentation?

[koliagogsadze_gmail.com]

sys/arm64/arm64/disassem.c

101–105

yes, it is from ARM documenation, for example https://developer.arm.com/documentation/ddi0596/2020-12/Base-Instructions/LDR--immediate---Load-Register--immediate--

LDR <Wt>, [<Xn|SP>, #<simm>]!
LDR <Wt>, [<Xn|SP>], #<simm>
LDR <Wt>, [<Xn|SP>{, #<pimm>}]

third pattern OP <RT>, <RN>, <RM> {, EXTEND AMOUNT } in comment was wrong at all, because we use addressing mode, see else branch of TYPE_02:

di->di_printf("%s\t%s, [%s, %s", i_ptr->name,
			    arm64_reg(sf, rt, rt_sp), arm64_reg(1, rn, rn_sp),
			    arm64_reg(option & 1, rm, rm_sp));

[koliagogsadze_gmail.com]

Removed helper function as disscused https://reviews.freebsd.org/D40967