MIPS: LLVMLinux: Fix a 'cast to type not present in union' error.

Remove a cast to the 'mips16e_instruction' union inside an if
condition and instead do an assignment to a local
'union mips16e_instruction' variable's 'full' member before the if
statement and use this variable in the if condition.

This is the error message reported by clang:
arch/mips/kernel/branch.c:38:8: error: cast to union type from type 'unsigned short' not present in union
                if (((union mips16e_instruction)inst).ri.opcode
                     ^                          ~~~~

The changed code can be compiled successfully by both gcc and clang.

Signed-off-by: Toma Tabacu <toma.tabacu@imgtec.com>
Signed-off-by: Daniel Sanders <daniel.sanders@imgtec.com>
Cc: Andreas Herrmann <andreas.herrmann@caviumnetworks.com>
Cc: David Daney <david.daney@cavium.com>
Cc: Manuel Lauss <manuel.lauss@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/9312/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>