target-arm: Eliminate unnecessary zero-extend in disas_bitfield

target-arm: Eliminate unnecessary zero-extend in disas_bitfield

For !SF, this initial ext32u can't be optimized away by the
current TCG code generator.  (It would require backward bit
liveness propagation.)

But since the range of bits for !SF are already constrained by
unallocated_encoding, we'll never reference the high bits anyway.

Reviewed-by: Peter Maydell <[email protected]>
Signed-off-by: Richard Henderson <[email protected]>
Message-id: 1441909103[email protected]
Signed-off-by: Peter Maydell <[email protected]>