testsuite/100272 - fix some malformed dg directives

The bug points out several malformed dg directives, the following
fixes the obvious ones where the testcases keep working after the
change.

2021-04-27  Richard Biener  <rguenther@suse.de>

	PR testsuite/100272
	* g++.dg/diagnostic/ptrtomem1.C: Fix dg directives.
	* g++.dg/ipa/pr45572-2.C: Likewise.
	* g++.dg/template/spec26.C: Likewise.
	* gcc.dg/pr20126.c: Likewise.
	* gcc.dg/tree-ssa/pr20739.c: Likewise.