V4 JIT: fix constant shifts.

V4 JIT: fix constant shifts.

Do not generate constant shifts of 0. We do not use the flags, so it's a
move. On ARM it's actually important not to do this, because lsr/asr
with imm=0 is a special case (shift of 32 bits).

When in the area, also skip generating an and of the second operand with
0x1f. For Intel this is done on the CPU, and for ARM the JSC assembler
will generate it for us.

This patch also updates the ARM disassembler to print the right
immediate values for the shifts.

Change-Id: I7c92c8d899352712c84e5534c48392d75466be0e
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>