Allow variable-sized temporary variables in gimplify.c

This is needed to build libgfortran for SVE.  The OpenMP code needs
to create temporary vector variables, and the variables will therefore
be variable-sized for SVE.  Earlier patches made such variables work.

2018-01-12  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* gimplify.c (gimple_add_tmp_var_fn): Allow variables to have a
	poly_uint64 size, rather than requiring an unsigned HOST_WIDE_INT size.
	(gimple_add_tmp_var): Likewise.

From-SVN: r256585