[SCEV] Increase FoldID bits size cover common cases.

This should fix the regressions introduced by a53d940 by making
sure no dynamic allocations are needed in the common case, while
retaining support for arbitrary SCEV expressions.

Alternative to D144335.

Compile-time impact:
* NewPM-O3: -0.11%
* NewPM-ReleaseThinLTO: -0.10%
*NewPM-ReleaseLTO-g: -0.08%

https://llvm-compile-time-tracker.com/compare.php?from=df016a9525e5722dfd1e1e1632cec3ed7b33bc8a&to=c1c64de4a973bcecaddbc4038a539234eb39413b&stat=instructions:u

Reviewed By: vitalybuka, nikic

Differential Revision: https://reviews.llvm.org/D144382

(cherry picked from commit eddecd3)

Author: fhahn

llvm/include/llvm/Analysis/ScalarEvolution.h

@@ -1297,7 +1297,7 @@ class ScalarEvolution {
   bool loopIsFiniteByAssumption(const Loop *L);
 
   class FoldID {
-    SmallVector<unsigned, 4> Bits;
+    SmallVector<unsigned, 5> Bits;
 
   public:
     void addInteger(unsigned long I) {