Fix constant island pass's handling of tBR_JTr. The offset of the instruction...

Fix constant island pass's handling of tBR_JTr. The offset of the instruction does not have to be 4-byte aligned. Rather, it's the offset + 2 that must be aligned since the instruction expands into:
        mov     pc, r1
        .align  2
LJTI0_0_0:
        .long    LBB0_14

This fixes rdar://8213383. No test case since it's not possible to come up with a suitable small one.

llvm-svn: 109076