[test, AArch64] Fix use of var defined in CHECK-NOT

LLVM test CodeGen/AArch64/aarch64-tbz.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 removes the definition and uses of variable to check each
line independently, making the check stronger than the current one. It
also removes unnecessary regex match for labels.

Reviewed By: dmgreen

Differential Revision: https://reviews.llvm.org/D99602