gcn: Fix __builtin_gcn_first_call_this_thread_p

Contrary naive expectation, unspec_volatile (via prologue_use) did not
prevent the cprop pass (at -O2) to remove the access to the s[0:1]
(PRIVATE_SEGMENT_BUFFER_ARG) register as the volatile got just put on
the preceeding pseudoregister.  Solution: Use gen_rtx_USE instead.
Additionally, this patch removes (gen_)prologue_use_di as it is then no
longer used.

Finally, as we already do bit manipulation, instead of using the full
64bit side - and then just keeping the value of 's0', just move directly
to use only s1 of s[0:1] and do the bit manipulations there, generating
more readable assembly code and better matching the '#else' branch.

gcc/ChangeLog:

	* config/gcn/gcn.cc (gcn_expand_builtin_1): Work on s1 instead
	of s[0:1] and use USE to prevent removal of setting that register.
	* config/gcn/gcn.md (prologue_use_di): Remove.

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