[PowerPC, test] Fix use of undef FileCheck var

LLVM test CodeGen/PowerPC/ppc-disable-non-volatile-cr.ll tries to check
for the absence of a sequence of instructions with several CHECK-NOT
with one of those directives using a variable defined in another.
However CHECK-NOT are checked independently so that is using a variable
defined in a pattern that should not occur in the input.

This commit changes occurence of the variable for the regex used in its
definition, thereby making each CHECK-NOT independent.

Reviewed By: NeHuang, nemanjai

Differential Revision: https://reviews.llvm.org/D99880