Fix LLVM implementation of TCG op_div2

The LLVM code implementing op_div2 and op_divu2 was broken: the dividend was bits*2 wide but the divisor was only bits wide. This fixes the problem by extending the divisor and then truncating the quotient.

In addition, an incorrect use of #ifdef TCG_TARGET_HAS_div_i32 was fixed (TCG_TARGET_HAS_div_i32 is always defined but may be 0 or 1).