V4 JIT: clean-up conversion functions and add toUInt32.

Change-Id: I7ac685145fa41db2a0e02c4d15d1d287d80621f8
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
V4 JIT: fix register allocator after change to s/uint32 usage.

Now that shifts can take a signed int32, reflect it in the check if a
binop will generate a call.

Change-Id: I3cab436bace31cdda327cf6132aa873b6c5456b1
Reviewed-by: Lars Knoll <lars.knoll@digia.com>