[openacc] Disable pass_thread_jumps for IFN_UNIQUE
If we compile the openacc testcase with -fopenacc -O2, we run into a SIGSEGV
or assert. The root cause for this is that pass_thread_jumps breaks the
invariant that OACC_FORK and OACC_JOIN mark the start and end of a
single-entry-single-exit region.
Fix this by bailing out when encountering an IFN_UNIQUE in
thread_jumps::profitable_jump_thread_path.
Bootstrapped and reg-tested on x86_64.
Build and reg-tested libgomp on x86_64 with nvptx accelerator.