[SCEV][NFC-mostly] Remove constant handling in TripMultiple computation

[caojoshua]

Summary

After landing more precise trip multiples in
https://reviews.llvm.org/D149529, the SCEV multiple computation handles
constants, so there is no longer any need for special constant handling
in getSmallConstantTripMultiple.

This patch can improve the multiple of a non-constant SCEV that is huge
(>=2**32). This is very rare in practice.

[caojoshua]

update commit message

[nikic]

LGTM

llvm/lib/Analysis/ScalarEvolution.cpp

 unsigned ScalarEvolution::getSmallConstantTripMultiple(const Loop *L,
                                                        const SCEV *ExitCount) {
   if (ExitCount == getCouldNotCompute())
     return 1;
 
   // Get the trip count
   const SCEV *TCExpr = getTripCountFromExitCount(applyLoopGuards(ExitCount, L));
 
+  APInt Multiple = getNonZeroConstantMultiple(TCExpr);
   // If a trip multiple is huge (>=2^32), the trip count is still divisible by
   // the greatest power of 2 divisor less than 2^32.
-  auto GetSmallMultiple = [](unsigned TrailingZeros) {
-    return 1U << std::min((uint32_t)31, TrailingZeros);
-  };
-
-  const SCEVConstant *TC = dyn_cast<SCEVConstant>(TCExpr);
-  if (!TC) {
-    APInt Multiple = getNonZeroConstantMultiple(TCExpr);
-    return Multiple.getActiveBits() > 32
-               ? 1
-               : Multiple.zextOrTrunc(32).getZExtValue();
-  }
+             ? 1U << std::min((uint32_t)31, Multiple.countTrailingZeros())
 
-  ConstantInt *Result = TC->getValue();
-  assert(Result && "SCEVConstant expected to have non-null ConstantInt");
-  assert(Result->getValue() != 0 && "trip count should never be zero");
-
-  // Guard against huge trip multiples.
-  if (Result->getValue().getActiveBits() > 32)
-    return GetSmallMultiple(Result->getValue().countTrailingZeros());
-
-  return (unsigned)Result->getZExtValue();
-}
-
 /// Returns the largest constant divisor of the trip count of this loop as a
 /// normal unsigned value, if possible. This means that the actual trip count is
 /// always a multiple of the returned value (don't forget the trip count could
 /// very well be zero as well!).
 ///
 /// Returns 1 if the trip count is unknown or not guaranteed to be the
 /// multiple of a constant (which is also the case if the trip count is simply
 /// constant, use getSmallConstantTripCount for that case), Will also return 1