MIPS/GAS: Simplify non-MIPS16 branch swapping sequence

MIPS/GAS: Simplify non-MIPS16 branch swapping sequence

Simplify non-MIPS16 branch swapping by copying the MIPS16 variant, which
sets the new position for the current instruction first and reduces the
calculation of the new position of the previous instruction.  Also refer
to previous instruction's frag and position via `delay' for consistency.

Reintroduce an explanatory comment, updated, previously removed with:

commit 1e91584932efd70020c8c98037d0cb93a0552a20
Author: Richard Sandiford <rdsandiford@googlemail.com>
Date:   Wed Mar 9 09:17:02 2005 +0000

<https://sourceware.org/ml/binutils/2005-03/msg00217.html>, ("Rework
MIPS nop-insertion code, add -mfix-vr4130 [5/11]").

	gas/
	* config/tc-mips.c (append_insn): Simplify non-MIPS16 branch
	swapping sequence.