[LoopUnroll] Clamp PartialThreshold for large LoopMicroOpBufferSize

The znver3/znver4 scheduler modules are outliers, specifying very
large LoopMicroOpBufferSizes at 512, while typical values for
other subtargets are on the order of ~50. Even if this information
is micro-architecturally correct (*), this does not mean that we
want to runtime unroll all loops to a size that completely fills
the loop buffer. Unless this is the single hot loop in the entire
application, the massive code size increase will bust the micro-op
and instruction caches.

Protect against this by clamping to the default PartialThreshold
of 150, which is the same as the default full-unroll threshold
and half the aggressive full-unroll threshold. Allowing more
partial unrolling than full unrolling is certainly non-sensical.

(*) I strongly doubt that this is actually correct -- I believe
this may derive from an incorrect reading of Agner Fog's
micro-architecture guide. The number 4096 that was originally
used here is the size of the general micro-op cache, not that of
a loop buffer. A separate loop buffer is not listed for the Zen
microarchitecture. Comparing this to the listing for Skylake, it
has a 1536 micro-op buffer, but only a 64 micro-op loopback buffer,
with a note that it's rarely fully utilized. Our scheduling model
specifies LoopMicroOpBufferSize of 50 in that case.

llvm/include/llvm/CodeGen/BasicTTIImpl.h

@@ -575,7 +575,13 @@ class BasicTTIImplBase : public TargetTransformInfoImplCRTPBase<T> {
     if (PartialUnrollingThreshold.getNumOccurrences() > 0)
       MaxOps = PartialUnrollingThreshold;
     else if (ST->getSchedModel().LoopMicroOpBufferSize > 0)
-      MaxOps = ST->getSchedModel().LoopMicroOpBufferSize;
+      // Upper bound by the default PartialThreshold, which is the same as
+      // the default full-unroll Threshold. Even if the loop micro-op buffer
+      // is very large, this does not mean that we want to unroll all loops
+      // to that length, as it would increase code size beyond the limits of
+      // what unrolling normally allows.
+      MaxOps = std::min(ST->getSchedModel().LoopMicroOpBufferSize,
+                        UP.PartialThreshold);
     else
       return;