powerpc/code-patching: Move instr_is_branch_{i/b}form() in code-patching.h

To enable moving selftests in their own C file in following patch,
move instr_is_branch_iform() and instr_is_branch_bform()
to code-patching.h

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/fca0f3b191211b3681020885a611bf73eef20563.1638446239.git.christophe.leroy@csgroup.eu