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

LLVM test CodeGen/AArch64/speculation-hardening.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 dependency between those CHECK-NOT by replacing
single occurence of the undefined variable by a regex match, and
multiple occurences by a definition followed by a use.

Reviewed By: aemerson

Differential Revision: https://reviews.llvm.org/D99866