Test: Add test case for bit_op convert folding

Test: Add test case for bit_op convert folding

Add test cases of all possible types of bit_op convert folding.
To check there is no tree dump like below:

   _5 = (uint8_t) _2;
   return _5;

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/bit_op_cvt.1.c: New test.
	* gcc.dg/tree-ssa/bit_op_cvt.2.c: New test.
	* gcc.dg/tree-ssa/bit_op_cvt.3.c: New test.
	* gcc.dg/tree-ssa/bit_op_cvt.4.c: New test.
	* gcc.dg/tree-ssa/bit_op_cvt.5.c: New test.
	* gcc.dg/tree-ssa/bit_op_cvt.6.c: New test.
	* gcc.dg/tree-ssa/bit_op_cvt.h: New test.

Signed-off-by: Pan Li <pan2.li@intel.com>