[Exegesis][AArch64] Use more generic cycles counter (#133376)

CPU_CYCLES counter does not work on some Aarch64 CPUs; CYCLES is more
generic and is equivalent to CPU_CYCLES in case the latter is supported.

Longer story - CPU_CYCLES work only on CPU models explicitly recognized
by libpfm4 ( via pfm_arm_detect_*() functions in
https://sourceforge.net/p/perfmon2/libpfm4/ci/master/tree/lib/pfmlib_arm_armv8.c
) and its name is consistent with ARM documentation. However, the
counter is architectural and is supported on all ARMv8 CPUs; libpfm4
recognizes generic PMU on unknown ARMv8 CPUs, but does not provide
CPU_CYCLES event. Instead, CYCLES is provided (an alias to
PERF_COUNT_HW_CPU_CYCLES). Physically, it is the same event with code
0x11. On supported architectures CYCLES also work, so the change should
not introduce regression.

Author: r-belenov

llvm/lib/Target/AArch64/AArch64PfmCounters.td

@@ -10,7 +10,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-def CpuCyclesPfmCounter : PfmCounter<"CPU_CYCLES">;
+def CpuCyclesPfmCounter : PfmCounter<"CYCLES">;
 
 def DefaultPfmCounters : ProcPfmCounters {
   let CycleCounter = CpuCyclesPfmCounter;

llvm/unittests/tools/llvm-exegesis/AArch64/TargetTest.cpp

@@ -65,7 +65,7 @@ TEST_F(AArch64TargetTest, SetRegToConstant) {
 }
 
 TEST_F(AArch64TargetTest, DefaultPfmCounters) {
-  const std::string Expected = "CPU_CYCLES";
+  const std::string Expected = "CYCLES";
   EXPECT_EQ(ExegesisTarget_->getPfmCounters("").CycleCounter, Expected);
   EXPECT_EQ(ExegesisTarget_->getPfmCounters("unknown_cpu").CycleCounter,
             Expected);