[PowerPC, test] Fix use of undef FileCheck var

Commit 6646033e removed the definition
of variable RESULT used in two CHECK-NOT directives in LLVM test
CodeGen/PowerPC/ppc64-i128-abi.ll. This commit replaces the uses by the
regex that was used to define that variable.

Reviewed By: nemanjai

Differential Revision: https://reviews.llvm.org/D99868