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

LLVM test Transforms/Coroutine/coro-split-sink-lifetime-O2.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 simplifies the CHECK-NOT block to only check for the
presence of any lifetime start marker since that is effectively what
the test was testing at the moment.

Reviewed By: junparser

Differential Revision: https://reviews.llvm.org/D99856