[X86] AMD Zen 3: cap LoopMicroOpBufferSize to workaround PR50384 (quadratic IndVars runtime)

While i would like to keep the right value here,
i would also like to be able to actually compile
e.g. vanilla test-suite.

256 is a pretty random guess, it should be pretty good enough
for serious loops, but small enough to result in tolerant
compile times for certain edge cases.

https://bugs.llvm.org/show_bug.cgi?id=50384

Author: LebedevRI

llvm/lib/Target/X86/X86ScheduleZnver3.td

@@ -35,7 +35,10 @@ def Znver3Model : SchedMachineModel {
   // The maximum capacity of the op cache is 4K ops.
   // Agner, 22.5 µop cache
   // The size of the µop cache is big enough for holding most critical loops.
-  let LoopMicroOpBufferSize = 4096;
+  // FIXME: PR50384: IndVars has quadradic complexity, with large values here
+  //        the compilation of certain loops ends up taking way too long.
+  // let LoopMicroOpBufferSize = 4096;
+  let LoopMicroOpBufferSize = 256;
   // AMD SOG 19h, 2.6.2 L1 Data Cache
   // The L1 data cache has a 4- or 5- cycle integer load-to-use latency.
   // AMD SOG 19h, 2.12 L1 Data Cache