[MIPS] Fix branch emulation for floating-point exceptions.

    
In the branch emulation for floating-point exceptions, __compute_return_epc
must determine for bc1f et al which condition code bit to test. This is
based on bits <4:2> of the rt field. The switch statement to distinguish
bc1f et al needs to use only the two low bits of rt, but the old code tests
on the whole rt field.  This patch masks off the proper bits.
    
Signed-off-by: Win Treese <treese@acm.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>