Add tree_fits_uhwi_p tests to BIT_FIELD_REF folder

Add tree_fits_uhwi_p tests to BIT_FIELD_REF folder

The first BIT_FIELD_REF folding pattern assumed without checking that
operands satisfy tree_fits_uhwi_p.  The second pattern does check this:

      /* On constants we can use native encode/interpret to constant
         fold (nearly) all BIT_FIELD_REFs.  */
      if (CONSTANT_CLASS_P (arg0)
          && can_native_interpret_type_p (type)
          && BITS_PER_UNIT == 8
          && tree_fits_uhwi_p (op1)
          && tree_fits_uhwi_p (op2))

so this patch adds the checks to the first pattern too.  This is needed
for POLY_INT_CST bit positions.

2018-01-04  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* fold-const.c (fold_ternary_loc): Check tree_fits_uhwi_p before
	using tree_to_uhwi.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@256258 138bc75d-0d04-0410-961f-82ee72b054a4