Fix failure in gfortran.dg/gomp/combined-if.f90 test

Fix failure in gfortran.dg/gomp/combined-if.f90 test

Enabling nvptx offloading results in extra '#pragma omp simd' statements
in the tree dump with an extra '_simt_'.

This is a backport from master
(commit 0fce12c0920c12492c878c0ca5f639f93b93f183).

2020-06-26  Kwok Cheung Yeung  <kcy@codesourcery.com>

	gcc/testsuite/
	* gfortran.dg/gomp/combined-if.f90: Adjust expected number
	of matches depending on whether nvptx offloading is supported.
	* lib/target-supports.exp
	(check_effective_target_offload_nvptx): New.