target-mips: fix branch in likely delay slot tcg assert

When a branch delay slot contains another branch instruction, the code
generated raises an exception, however since is_branch==1,
handle_delay_slot() doesn't get called immediately. This means
ctx->bstate isn't set to BS_BRANCH, and the decoder continues decoding
until a non-branch instruction is found.

If the first branch was a branch likely instruction then each
instruction after it generates code for the unlikely case, to go to the
next tb starting after the delay slot. This results in multiple goto_tb
tcg ops being generated with the same exit number. When debug is enabled
this hits:

tcg-op.h:2589: tcg_gen_goto_tb: Assertion `(tcg_ctx.goto_tb_issue_mask & (1 << idx)) == 0' failed.

This is fixed by removing is_branch entirely, and calling
handle_delay_slot() if (ctx.hflags & MIPS_HFLAG_BMASK) was set prior to
the current instruction being decoded. This still prevents
handle_delay_slot() being called immediately after a branch but allows
it to still be called after a branch within a delay slot.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Yongbok Kim <yongbok.kim@imgtec.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
