[PowerPC] Remove asserts from the disassembler.

[PowerPC] Remove asserts from the disassembler.

My previous patch had added a couple of asserts to the disassembler.
The problem with this is that the disassembler is not just used for the
text section it is also used to disassemble the data section of an
object where the bytes do not necessarily represent instructions. If the
data in the data section happens to look like an illegal instruction
then llvm-objdump will assert on data because it is finding an illegal
instruction that is not actually an instruction at all.

Reviewed By: nemanjai, #powerpc

Differential Revision: https://reviews.llvm.org/D149711