[PowerPC, test] Fix use of undef FileCheck var

LLVM test CodeGen/PowerPC/ctrloops-softfloat.ll tries to check for the
absence of sequences 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: nemanjai

Differential Revision: https://reviews.llvm.org/D99881