openmp: Scale type precision of collapsed iterator variable

openmp: Scale type precision of collapsed iterator variable

This sets the type precision of the collapsed iterator variable to the
sum of the precision of the collapsed loop variables, up to a maximum of
sizeof(long long) (i.e. 64-bits).

2021-03-01  Kwok Cheung Yeung  <kcy@codesourcery.com>

	gcc/
	* omp-expand.c (expand_oacc_for): Convert .tile variable to
	diff_type before multiplying.
	* omp-general.c (omp_extract_for_data): Use accumulated precision
	of all collapsed for-loops as precision of iteration variable, up
	to the precision of a long long.

	libgomp/
	* testsuite/libgomp.c-c++-common/collapse-4.c: New.
	* testsuite/libgomp.fortran/collapse5.f90: New.