On 64-bit target, prefer using RDPC over CALL to get the value of %pc.
This is faster on modern processors (Niagara T1 and newer) and avoids
polluting the processor's predictor state.
The old behavior of using a fake CALL is still done when tuning for
classic UltraSPARC processors, since RDPC is much slower there.
A quick pgbench test on a SPARC T4 shows about 2% speedup on SELECT
loads, and about 7% speedup on INSERT/UPDATE loads.
Reviewed By: @s-barannikov
Github PR: https://github.com/llvm/llvm-project/pull/78280
118d4234ac