Improve Thumb disassembler for LDR/STR/PUSH/POP/BKPT.

Disassemble 16-bit Thumb PUSH, POP, BKPT.

Clean up 32-bit load/store to handle all cases (including
previously unrecognized indexed load/store) in one place;
this also fixes LDRSH erroneously disassembled as LDRSB.

Recognize more UNDEFINED instructions and other minor
cleanup.

Change-Id: Ifdd177745b70e3f774cc0469deb81191b035f51b