target-arm: Fix UNDEF cases in Thumb load/store

Decode of Thumb load/store was merging together the cases of 'bit 11==0'
(reg+reg LSL imm) and 'bit 11==1' (reg+imm). This happens to work for
valid instruction patterns but meant that we would not UNDEF for the
cases the architecture mandates that we must. Make the decode actually
look at bit 11 as well as [10..8] so that we UNDEF in the right places.

This change also removes what was a spurious unreachable 'case 8',
and correctly frees TCG temporaries on the illegal-insn codepaths.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
