testsuite: Fix up gcc.target/i386/pr99726.c test [PR99726]

testsuite: Fix up gcc.target/i386/pr99726.c test [PR99726]

-m32 shouldn't be used in gcc.target/i386/ testcases, people do
test with -m32/-m64 to get 32-bit compilation tested.
And, -floop-nest-optimize is a graphite optimization, so might not
be enabled in all gcc builds.

2021-04-02  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/99726
	* gcc.target/i386/pr99726.c: Remove -m32 from dg-options.  Move
	-floop-nest-optimize to dg-additional-options guarded on fgraphite
	effective target.