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

LLVM test Transforms/HardwareLoops/ARM/structure.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 llvm.loop.decrement.i32 which
rules out the presence of the whole sequence and does not involve an
undefined variable.

Reviewed By: dmgreen

Differential Revision: https://reviews.llvm.org/D99591