MIPS: Correct MUSTBE32 interpretation in delay slot handling

MIPS: Correct MUSTBE32 interpretation in delay slot handling

This change addresses `micromips_instruction_has_delay_slot' and
`mips16_instruction_has_delay_slot' that both incorrectly interpret
their MUSTBE32 argument.  Their callers assume that when the flag is
clear these functions will return 1 when any non-compact jump or branch
instruction is present at ADDR, while in fact they will only return 1
for 16-bit such instructions only.  This change makes the implementation
match the expectations.

	* mips-tdep.c (micromips_instruction_has_delay_slot): When
	!mustbe32 also return 1 for 32-bit instructions.
	(mips16_instruction_has_delay_slot): Likewise.  Add an
	explanatory comment.