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

LLVM test Transforms/LoopVectorize/pr34681.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 only checks for the absence of icmp ne 1 which rules out the
presence of the whole sequence and does not involve an undefined
variable.

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D99582