Fix BXJ is undefined in AArch32.

BXJ was incorrectly said to be unsupported in ARMv8-A. It is not
supported in the A64 instruction set, but it is supported in the T32
and A32 instruction sets, because it's listed as an instruction in the
ARM ARM section F7.1.28.

Using SP as an operand to BXJ changed from UNPREDICTABLE to
PREDICTABLE in v8-A. This patch reflects that update as well.

This was found by MCHammer.

llvm-svn: 235024