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

LLVM test Transforms/InferFunctionAttrs/annotate contains two RUN
invokation (UNKNOWN and NVPTX lines) which involve a CHECK-NOT directive
with a variable not defined by the enabled CHECK prefixes. This commit
fixes that by:

- enabling CHECK prefix for unknown target with specialisation when it
  differs from other targets
- checking for absence of bcmp with any attribute for NVPTX

Reviewed By: tra

Differential Revision: https://reviews.llvm.org/D99589