target-mips: fix hflags modified in delay / forbidden slot

All instructions which may change hflags terminate tb. However, this doesn't
work if such an instruction is placed in delay or forbidden slot.
gen_branch() clears MIPS_HFLAG_BMASK in ctx->hflags and then generates code
to overwrite hflags with ctx->hflags, consequently we loose any execution-time
hflags modifications. For example, in the following scenario hflag related to
Status.CU1 will not be updated:
    /* Set Status.CU1 in delay slot */
    mfc0  $24, $12, 0
    lui   $25, 0x2000
    or    $25, $25, $24
    b     check_Status_CU1
    mtc0  $25, $12, 0

With this change we clear MIPS_HFLAG_BMASK in execution-time hflags if
instruction in delay or forbidden slot wants to terminate tb for some reason
(i.e. ctx->bstate != BS_NONE).

Also, die early and loudly if "unknown branch" is encountered as this should
never happen.

Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>
