[SCEV] Increase FoldID bits size cover common cases.

[SCEV] Increase FoldID bits size cover common cases.

This should fix the regressions introduced by a53d940cee6f 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