[OpenMP, test] Fix uses of undef S*VAR FileCheck var

Fix the many cases of use of undefined SIVAR/SVAR/SFVAR in OpenMP
*private_codegen tests, due to a missing BLOCK directive to capture the
IR variable when it is declared. It also fixes a few typo in its use.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D99770